It looks like I underestimated that suggestion.

Having played a little more with MOBAC, it's only taken a few days to get the full UK in 1:25000 in JNX format. The vast majority of that was the PC's just getting on with the process of downloading and converting. I think it would have taken longer and more effort to make it out of my Memory Map maps, and this way I have the latest maps too (my MM maps are 2011).

Thanks Sergio2!