Hello Les,
The problem as you describe it in post #565 seems similar to something I observed with a friend's Hornby Duchess pulling a heavy rake of Pullman Coaches on Sunday. His problem was with the tender coupling, the type where the NEM socket is a detachable part located by a triangular wedge into a recess of the same shape on the underside of the tender. The small tension lock coupler clips into the NEM pocket as normal. We found that the top and underside of the NEM socket were of slightly different thicknesses and as originally fitted 'thick side up' was setting the coupling too low for satisfactory coupling to the lead coach. Simply unplugging the tension lock coupler from the NEM socket, re fitting the socket 'upside down' that is thin edge against the base of the tender, and re fitting the coupler, cured the problem.
I don't know what the coupling arrangement is on your B1, but if it is similar to the Duchess tender this may be a pointer for you.
Good luck,
John