any reason why my one year old lab should suddenly start weeing and messing overnight. I also left him for a couple of hours this morning and he had weed . Usually he is good to be left for long periods during the day.

This really needs to be checked by a vet to ensure there is no medical reason.  Male dogs will often start this if a bitch comes into season nearby.......

i am not aware of any bitches near us. Do you think it could be anything to do with the cat who has now decided to come inside because the weather is colder?

It's possible, but don't be deceived into thinking 'nearby' is next door - nearby could be half a mile away!