Thank you all. I did try the vet but they only had emergency apps and it's not that. Based on his other symptoms, a friend who is a dog professional thinks his anal glands might be blocked. He's now eating sardines in olive oil with some carrot and tucking into it nicely. I will get some liver too for later and offer him food little and often tonight. Vets tomorrow though if still no poo.
I took dog to the vet yesterday who checked his anal glands (not impacted but she released them anyway if that's the correct terminology). He has been eating since Tuesday evening, small amounts of sardines, offal and cooked chicken. Drinking and weeing ok but still no poo as far as I'm aware (the garden is too dark and the torch too feeble to check but nothing on any walks. Whe do I go back to the vet do you think? He seems ok in himself. Not overly sprightly but then he is a very chilled dog normally. TIA