Thanks for the information everybody.
I already have an LPL 6700 enlarger for everything up to 6x7 so any problems with a 50mm lens on 35mm film don't really matter. I will build this enlarger just for 4x5 (or 5x4 as we call it over here - no idea why it's different!). I will then make negative holders for 6x9, and 6x12 as well.
Initially I will use a diffused light source and include a tray for contrast filters but I may play around with blue and green LEDs as well.
I have done some initial tests using a negative taped to a small Jessops 5"x4" light box and a 105mm lens and it seems promising. Somewhere I have a Wray 4 1/2" enlarging lens but I cant find it at the moment. The 105mm lens seems to show all of the negative though so will probably be fine for my initial tests.
I have the bellows and focussing rack from a long deceased 9x12cm camera so all I need to work out now is a base, column and some way of moving the enlarger head up and down the column. I am thinking of using a square section with register pins with holes every 1/2" or so. Not as convenient as a wind up and down mechanism but should be useable, especially if it is only used for 4x5 on standard paper sizes.