Well having dug out the workshop manual and read the right bit I found that if I put the fork legs into the lower clamp and clamped up (with 190mm beyond top of lower brace), I could then push the top brace on from above very easily. It had to be pulled on a little to come down straight on the stem, but not a lot. So reassembly in this order - essentially pushing top clamp down onto legs rather than trying to force legs up through both clamps, made all the difference. Next time, read workshop manual first!