Many thanks for advice Jo, Fee and Jacky.
I had never come across this sort of thing with dogs I have owned before, so it is sort of a relief to hear that other people have the same problem.
I don't think toys would distract him as, although he likes them when he is bored, they come quite far down his list of interests, with walks way ahead of anything else. And I would be rather reluctant to put a muzzle on him as he pants immediately when he gets excited and I think he might explode if he couldn't open his mouth properly. I am not so worried about him biting a child as he is very gentlemanly about picking his targets and doesn't go for anything defenceless - if PV, the Chihuahua, decides he wants to come on the walk, Rupert never terrorises him, only me and Scooter. And we, also, are out in the sticks and rarely come across other people when walking.
I have a feeling that, if I could stop him barking, the problem might be solved, and I wonder about putting one of those collars on him before we go out, either the ones that squirt citronella or, someone has suggested, one that gives a mild electric shock - I know he would hate the latter as he once blundered into an electric fence when he was chasing a rabbit, he made a huge fuss and was upset for hours afterwards. I think they are quite expensive so I would like to be sure they would work before shelling out.
Any further thoughts?