Have to report an epic fail on this . Cartridge Bottom bracket fitted snugly and was ordered at 115mm as suggested for new crankset , no problems fitting it , so we were correct about size 68 and british thread . Found a matching crankset for the cottered crank in square taper 52t/42t here and ordered it

Silver Double Alloy Road Race Chainset 42 52T RRP £34 95 | eBay


Waited giddly for postman to arrive , matched the new crank against the cottered and all dimensions seemed perfect , then put the cranks on and ujnfortunately they are way too far off out from the frame. The old cottered chainset hugged it with a few mill to spare , had over a cm here with new crank,chainline looked way off and deraileur could barely stretch . Strange as my worry was that the new crankset would be too close as I was going down from a 136mm cottered bb to a 115mm square taper . Plan now is to send back the ill fitting crank and just struggle on with the cottered . It seems that this kind of crank conversion would be a lot of costly trial and error as the cranksets vary so much in depth as they sit on the square tapers .