Thanks Chaps,
Through the help of Mozz and everyone else I am going to send the BECM off to Rick the Pick with the new key to get it all sorted once and for all!
Rick tells me he can also set the BECM up so it immobilises once the key unlocks the door which future proofs the issue.
I feel I was unnecessarily harsh on Yeovil LR in my frurstrations and they have delivered the goods regarding the information required of the new key to enable me to get it recoded to the current BECM.
Quite a saga but good to know there's people out there willing to be so helpful!