I think I'd be inclined to have him vet checked as the problem has started since he was castrated. Hopefully a check will rule out any physical problem caused by the surgery.
When you let him out last thing at night do you watch him to make sure he has a pee? If he's just wandering about and not peeing that could be the reason for the night time peeing. Feeling cold or hot at night could be another reason, if it's making him wakeful and restless.
You could also try getting up at say 6.00ish for a few mornings and letting him out (hoping he hasn't peed by then), then go back to bed. If that works you can then try letting him out for a pee a bit later every morning and hope it gets him back into his previous routine eventually.
Also, when he pees indoors don't use bleach to clean up because the ammonia in it smells like pee and may encourage him to use the same place again. use either a specialist pet odour and stain remover or a solution of bio laundry liquid or powder. The enzymes help to eradicate odour as well as stains.
Hi Elaine
Many thanks for your reply and apologies for late response - Christmas seemed to take over a bit!
Yes, we take him out and stay with him last thing at night. It can take up to 15 minutes to get him to pee! We use the same words we used when toilet training him when he first arrived so he associates the two things hopefully.
Good idea regarding getting up a bit earlier. We will give that a go and see how we get on.
I'm really hoping this problem wasn't caused as a result of his neutering but it does seem odd. Ironically, he was the easiest of our cockers to house train right from a very small pup! So it seems particularly ironic we're now experiencing these problems!
Thank you for the tip about not using bleach too. I tend to use the spray Detox or plain disinfectant but will give the biological laundry liquid a go too.
As for being too hot/cold - the heating is not on overnight as it goes off at 10.30pm coming back on again at 6am. I'm not really sure how I would be able to tell if the temperature was wrong for him though? It feels okay to us but then again we don't have fur coats! Food for thought though.
I do hope we can get to the bottom of this and thank you for you helpful reply.
Hope you've had a Happy Christmas?