Well, Dave's started spraying again. I was reading the paper this morning and, out of the corner of my eye, caught him spraying against the living room wall - about 3 feet away from where I was sitting.
The only two things I haven't yet tried are medicating him and getting a behaviourist in - so on Monday morning I'm going to ring the vet, get him an appointment just to make absolutely 100% sure there's no medical reason why he's spraying. If he checks out ok medically, which I think he will, I'm going to ask for a referral to a behaviourist. I wouldn't medicate him to stop the behaviour, I'd rather see him happy and 'with it' in another home.
I'm working on the principle that at least if his behaviour IS due to what he sees as too much company, CP would be able to rehome him with the information that his behaviour is definitely due to an issue with other cats and so would [most likely] not be an issue if he were an only cat.