I had smart meters fitted a few months back and they changed the tails, they look to me to be 25mmsq even though I have a 60amp master fuse. It would make sense to fit 25mmsq tails when fitting smart meters as the push for Evs will render 60 amps insufficient.
Looking at my tails ๐ฎ the outer sheath looks to be marked 25mm so if I were you I'd ask your electrician to check, I'd expect him to know.
Presumably UK Power want some sort of official confirmation rather than you saying they're 25 mmsq.
I would expect upgrading to a 100 amp fuse to be a fairly common request now, but I think you're hitting a common problem now where many employees don't have sufficient skills/product knowledge to do anything outside of the basic tasks.
However, @Beki_EONNext has said that it's something that they can do, and free of charge, so hopefully she'll come back to this thread and help you out as simply calling eon-next doesn't look like it's going to work.