Oh dear, poor Merly in the wars again, fairly serious this time...
We noticed he has been putting on weight since the summer, despite having the same amount of food and exercise and over the past few days he has been dripping urine every now and them. On reflection, it seemed to me that he has been less interested in female dogs than normal, and I suddenly thought that his testes were looking smaller than usual! Normally he has large, smooth, egg-like testicles (sorry!), now they look sort of shrunken and small... OH took him to the vets yesterday - he had gained 2kg!!!

But a urine test showed he does not have a bladder infection.
I took him in again this morning for an ultrasound. Poor lad has an enlarged prostrate, and both of his testes are smaller than they should be, one of them has some kind of little lumps or cysts showing in the ultrasound. In addition, his nipples are slightly swollen, so it seems that his testosterone levels have dropped and estrogen levels have risen.
I had already guessed what the advice would be - neutering. I and OH are both sad about this, but I would much rather go ahead and have him neutered, as this will prevent any further deterioration in the testes and also his prostrate should go back to its normal size afterwards. I have booked him in for 10 November, at 08:00 in the morning.
Feeling a bit blue, really, but hopeful this will sort out the problem. Oddly, of the four male labradors I have had in my life, he will be the third who has suffered prostrate issues - I understand it is very common in intact males. With Bones, it was treated by chipping him, which seemed to sort out the issue, but the vet says that this is not possible with Merlin, since his testes are not in a good state. She said this is not an emergency, but she really advised neutering him.