Thank you for your reply.
My dog is a three year old male, rescue poodle. We have had him for three weeks now. He was neutered.
I imagine that he has been doing this a while, the family who fostered him were lovely but I suspect they haven't been completely honest about his behaviours. It could be why he ended up in a shelter in the first place.
I try to reassure him by stroking him (which I realise may be reinforcing the unwanted behaviour). It doesn't seem to make any difference whether he is on a lead or not, but there are certain people that he will always bark at, men in hats, with hi-viz jackets, builder/workmen types, but it does seem to be getting worse, and once he has started he then will sometimes bark at everyone. I have found saying 'No' very firmly does work.
Sometimes I do tense up when I see these people walking towards us which may not help. Sometimes I tell him not to bark and he listens surprisingly, but not always.
He does not bark all the time, but it still does seem a lot, and my neighbours are complaining about him barking at night.