Your electrician is talking drivel. Most DNO cables are 10mm split concentric. DNOs work to their own rules with regards to cable sizes and I've never seen 25mm on a domestic property Unless you are planning to charge your EV during the day instead of taking advantage of off peak tariffs,I doubt you will have any problems with a 60A main fuse. The fuses used don't just blow at bang on 60 Amps anyway, how quickly they blow depends on how overloaded they are, and that means they can tolerate more than 60A for quite a while sometimes. I would talk to your DNO about possible concerns your main fuse holder is old and therefore potentially dangerous - should get a free upgrade that way.