Cat drinking lots of water (more than usual)

Our cat suddenly seems to be drinking lots of water. He is a one year
old male ginger cat and since a kitten he has always drank from a mug.
He has recently started to drink a good cm or 2 of water a day from the
mug (big round one). He eats a mixture or dry and wet food (quite a
greedy cat). Not sure why he is suddenly drinking more (used to drink
that amount over a couple of days) - he has also been sick several
times this week as well. On one day in the morning he was fairly dopey
(wouldn't eat (very unusual, usually wakes us up for food) or play) but
perked up later that morning!

I havn't taken him to the vet yet but am worried about him. Anyone any
ideas - seems a bit early for any kind of kidney failure or major
problems and can't believe diabeties can come on within a few weeks
(other issues i have seen related to excess water drinking). His fur
seems quite healthy and he seems to wash himself a fair bit.

Can't think of much else tosay that might help, he has one other cat
who is also a male who tries to come in through our cat flap and also a
female cat next door who he plays with quite a bit. He has had all his
jabs and has been de-maled. He is also quite playful and affectionate
except if you go near his back legs (probably another problem), he is
quite happy with you playingand holding his front paws.

The weather has got alot colder recently, but it has been fairly dry
outdoors. Not sure if that has an effect?

Any help would be appreciated - shall I take him to the vet or see how
things pan out? We have insurance so if it is major they should pay up.

Why not take him to the vet and have have him checked out, just to be
sure. His increased water consumption might be nothing. My cats drink
more at this time of year, I think because the air gets so dry and they
just need more water in order to remain hydrated. Still, why not take
him to the vet...just to be sure? If it is something more serious, you
might be glad you caught it when you did.

Steve