Walter,
OK I'm Navy and not RAF, but I thought I'd given you a few hints. It isn't sloppy to put a waypoint of 'roughly' the Mull light house in the TANS. The flight is VFR, and all you want is to be pointed at the Mull, which you'll identify visually, change waypoint and fly on to the next, along the coast.
It would never be one's intention to actually overfly the waypoint and straight line it to the next.
Their 'intended flight plan' died with them, as it would be constantly updated as the trip progressed.
As for a portable TACAN, I'm 99.9% certain they would not have use of such. Whay would they go to all that expence and bother?
It was a VFR flight. There is no proof that at any time they intentionally lost VMC and elected to continue IFR.
Why they crashed is, and will remain, unclear. They have not been found to be negligent to the required standard. They should be cleared. NOW.
Oh, and psyclic, if you listen to people like K52, they were negligent because they didn't have time for breakfast. Clutching at straws, or what!