That's true although if it was just a bit more than the usual amount of water I'd be OK, it's just that it's so much more!! But I've made a vet appointment for tomorrow morning to get them checked over just in case (the receptionist said it was probably just because they're warm having been outside, like others on here said, but agreed it was good to check just in case). Will let you know what they say! I'm also going to ask about litter training because I've just discovered a massive wet patch on my sofa
Personally, I find a thick layer of newspaper and then hay is the best for litter trays, as rabbits do like to pee, poo and eat in the same place! I know you changed over from that, but wonder whether you could put the litter in the trays and then cover with thick layer of hay, which may encourage them to use their trays. Also, when cleaning out the trays, put a small amount of their poo back in.
It might be worth restricting them to a large dog crate with run attached (lots of ideas on here) when you're not with them, eg overnight, at least until they have settled in and know they have their own territory.