Oh Hitch I feel like such an idiot! This is exactly what the issue was - the fonts look fine if I switch to Voyage view!! So sorry to waste your time. I had a lot of experience with ebooks about 12 months ago but have not done much in last 12 months so completely forgot about this and in the rush before xmas I was only checking in the "fire view".
I've just linked here to same/main discussion you had going with my collegue about this on Amazon KDP site:
https://kdp.amazon.com/community/thr...08185􎣙