Thanks for replies - much appreciated.
I had a direct message from Ian C who (amongst other useful info) mentioned a recessed lens board.
So I searched t'interweb and saw that KHB sold them for my LPL enlarger - just needed to check which size mounting thread I needed - I went into garage to check and saw that I had a lens board that was kinda recessed, only a cm or so, but I had the recess "pointing downward" - so I reversed it and that made a big difference - easily printed a 35mm neg at 12x16 using the 50mm lens.
I'lll have another go using 16x20 next weekend - if necessary I'll buy the more recessed lens board from KEH, and whilst I ordering that I might as well purchase the extended LPL focussing cable which I have been thinking about for ages !
Once again thanks for all replies - have been super helpful.