In my earlier post I mentioned Arctic's splendid How to relating to BCU removal. If you do happen to go down that route I would also point you at another helpful post Arctic post relating to removal of the BCU plugs. This is to be found at post #6 in thread
https://the75andztclub.co.uk/forum/s...d.php?t=249323.
THE hard plug to remove is the biggest one - which has an unusual 'Sliding' mechanism - the main body of the plug into which the many wires stream in is surrounded by a three sided Shell. This Shell has to be moved out , over the edge of the BCU , in a direction away from the other two plugs. Arctic shows a picture in which he inserts a screw driver close to the main wires body to achieve this sliding. Alternatively , after pushing the sliding surround slightly over the BCU edge , you can then slip a screw driver up the side of the BCU , into the overhanging Shell and lever that way ( I did this).
By the way , I know Jamiewelch can supply a new PCB for a good price, he may well be able to supply an entire key ( all parts) at a competative cost.
All the best