He's also met a bunch of other dogs large and small, on and offlead (with muzzle), culminating in being roundly told off by a bossy female Jack Russell terrier yesterday - with no attempt at retaliation. He does chase excessively and has to be intercepted if he comes across a dog that is nervous, and obviously that won't do. I think he thinks he's playing, but if the other dog is scared and he won't let them go, we can't have that.
It looks like he has a specific beef with black and white male springers and sprollies and similar crosses: these are the only dogs where he starts growling and lunging rather than just being clueless, over-enthusiastic and generally socially inept. Liver and white female springers are fine (I've not yet found a male liver and white springer, so I'm not sure if it's the sex or the colour he objects to). Black and white female cockers are fine.
On the whole, he seems to be happier around bitches (though not exclusively : he liked Sid the greyhound a lot, and of course he also likes Az.)
He's put on a lot of weight, so it's time for what one of the Oldies volunteers calls 'The Knutoff Procedure'. With a bit of luck this will make him a little less pushy around male dogs in future, as well as preventing the birth of yet more unnecessary lurchers. He's booked in at the vet for the operation on Monday.