Thanks for the advice. In the end it just needed a bit of extra force to get it past a tight spot. Now goes onto the hub fine so I can have sensible gearing (42-52 with 14-16-18-21-25) rather than the 47-53 and 13-15-17-19-20 that the bike came with…Well I’d say if it’s screwed on nearly all the way you have matching threads so maybe bottom threads just need a thread file or needle file run over them just a tad as slight bruising may have occurred along the way..or a little grinding paste so you can work the freewheel back & forth so it seats properly..I’ve done this in the past with no issues with the threads matching of course….just take your time.
Others may have a different solution.