Gwynedd

The crew was performing a flight from the RAF Llandwrog (Caernarfon Airport) and while cruising by night over the Snowdonia mountains, the twin engine aircraft hit the slope of Mt Mynydd Perfedd (812 meters high). The aircraft was destroyed by impact forces and all four occupants were killed.
Crew (9th AFU):
Sgt John George Shepherd, pilot,
Sgt Edwin John Keightley, wireless operator and air gunner,
LAC John Chrystal, navigator,
LAC John Thomas Key, navigator.
Source and photos:
http://www.peakdistrictaircrashes.co.uk/pages/wales/walesperfedd.htm

While cruising by night at a height of some 2,000 feet, the twin engine aircraft hit the east slope of Mt Drum, in the Snowdonia. All five occupants were killed.
Crew (2nd AFU):
Sgt R. W. T. Smith,
P/O J. M. Polomark,
P/O G. Williams,
Sgt W. Jackson,
Sgt N. V. Robson.

En route, while cruising in a snowstorm, aircraft hit Mt Lyn Cowlyd located some 15 km southeast of Bangor, Wales. The pilot was killed while all three other occupants were injured.
Crew:
Sergeant Joseph William Francis Grant, pilot, †
Sergeant Robert Birch, navigator,
Warrant Officer Thomas Renni Renton, wireless operator,
Mervyn John Byrne, wireless operator.

The crew was completing a training flight in the region of Barmouth, Wales. While cruising at a height of 1,000 feet, the twin engine aircraft hit the slope of Mt Bwlch y Llan and was destroyed by impact forces. All 4 occupants were killed.
Crew (9th AFU):
P/O Frederick Roger Bullock, pilot,
P/O John Alexander McIntosh, navigator,
P/O John William Butcher, navigator,
Sgt Henry John Noy, wireless operator and air gunner.

The crew were on a night cross country navigation exercise from their home base of Jurby, on the northern plain of the Isle of Man. Most routes flown from Jurby were circular tours of the Irish Sea. While flying west, presumably on a clockwise orbit of the Irish Sea, the aircraft flew into to gentle grassy slopes of the Carneddau. At about 12:30 on the 1st December the RAF Mountain Rescue team based at Llandwrog were informed by the station's Flying Control Officer that two of the aircraft's crew had walked down to Bethesda, after having spent the night in the aircraft. The MRT began their search from Bethesda police station, they were then directed to the village store where they found the two aircrew. The two crew were unable to avail the team members of the exact location of the aircraft, having walked down in low visibility. They then began a search of the area to the north and west of Carnedd Llewellyn but found nothing before sunset, when they had to abandon the search and return to base. The second day of the search started at 07:30 with the team searching around Carnedd Dafydd. At 11:00 a third member of the crew was reported as having walked off the mountain to Bethesda. He was able to inform the Police more about where the aircraft, and the now lone fourth crewman was located. The hill parties were recalled and then drove to Tal-y-bont before proceeding up the track to Melynllyn to search the mountains in that area. At about 15:30 one of the search parties came across the wreck in the area between Foel Grach and Garnedd Uchaf and the cliffs of Craig y Dulyn. The fourth crew member was found alive in the aircraft's turret. He was reported to be "suffering from dehydration, starvation and a fractured foot, but his spirits were high". He was recovered by a stretcher party, arriving at the Ambulance after nightfall.
Crew (5th AOS):
Sgt James A. Knight, pilot,
Sgt Roy Gilbert, wireless operator and air gunner,
LAC Reed, navigator,
LAC Thomson, navigator.
Source:
http://www.peakdistrictaircrashes.co.uk/pages/wales/walesef909.htm

During a training flight from RAF Penrhos, the twin engine aircraft hit the slope of Mt Craig Cwm shrouded in cloud and located southeast of Penygroes. All five crew members were killed.
Crew (9th AFU):
Sgt Jozef Szczerbaty, pilot,
F/Sgt James Davies, air gunner,
AC2 Ronald Ross, air gunner,
AC2 Philip Andrew Shields, air gunner,
AC2 Herbert Henry Sutton, air gunner.

The aircraft and crew were stationed at RAF Halfpenny Green to the south west of Wolverhampton. During the evening of the 8th November 1943 to crew of five took off for a night navigation exercise. The route was to have covered ~320 miles passing over of close to the following points Base - Rhyl - Conwy - Douglas - Maughold Head - Chetton - Base. A short way into the flight the crew received a QDM from RAF Tilstock of 0500 with the location of the aircraft being assumed to be in the area of 4 miles NW of Shawbury roughly on course for Rhyl. It was next plotted by the Royal Observer Corps on a route that appeared to be a direct track from Wrexham to Conwy. A direct track from the Wrexham area to Conwy would not normally have taken an aircraft into Snowdonia, but on the night there was a 15 mph wind blowing from west north west which the Court of Inquiry remarked on as "if correction for drift was not taken the aircraft would drift south of the second turning point". At around 20:30 the ROC observed a fire near Carnedd Dafydd, shortly before this the aircraft while flying in a north westerly direction had impacted the upper slopes of Pen yr Ole Wen. The aircraft's engines had been torn off and rolled almost 1000 feet down the mountain and the fuel tanks had ruptured with their contents igniting. The fuselage was destroyed but was not burnt by the fire.
Crew (3rd AFU):
Sgt Ernest Andrew Hoagg, pilot,
Sgt John Hedgley Lewis, navigator,
Sgt William Gavin, bomb aimer,
Sgt Leslie John Hill, wireless operator and air gunner,
Sgt Douglas John Roberts, wireless operator and air gunner.
Source: http://www.peakdistrictaircrashes.co.uk/pages/wales/walesn9855.htm

The crew left RAF Bishops Court for a night training exercise. While flying over the north of Wales by night and marginal weather conditions, the aircraft hit the east slope of Mt Mynydd Perfedd and was destroyed. All four crew members were killed.
Crew (7th AOS):
Sgt John George Shepherd, pilot,
Sgt Edwin John Keightley, wireless operator and air gunner,
LAC John Chrystal, navigator,
LAC John Thomas Key, navigator.

The crew was performing a gunnery exercise from RAF Mona. While flying along the coast, the plane tried to fly between the shore and a large hopper which linked the Granite Quarry on the Slopes of Gyrn Ddu. This quarry was linked to a jetty by a transporter cable to support the cars from the mine workings above the village. The Anson flew into the cable, went out of control and crashed into the sea 100 yards offshore. Three crew members were killed while three others were injured.
Crew (3rd AGS):
Sgt R. W. Eastman,
F/O Harold Stevens, pilot, †
Sgt Ivor Ormondroyd, air gunner,
Sgt T. H. Jeffs, air gunner, †
Ronald Albert Cashen, †
John White.
Source: http://aviation-safety.net/wikibase/wiki.php?id=25175

While cruising at its assigned altitude, the twin engine aircraft broke in mid air and crashed in a farmland located in Caernarfon. All four occupants, two pilots and two ATC cadets, were killed.
Crew (9th AFU):
P/O Laurence A. Walker, pilot,
Sgt James Winchester, wireless operator.
Passengers:
Geoffrey A. Stapleton,
Geoffrey Foulkes.