This is a pretty big job (by our standards, anyway!) We're talking five figures. If he walked away now, he'd lose a lot of money and we're not talking hundreds. He's had scaffolders do the scaffold, then between two and four men working two days hacking off the render, and then three of them putting on the scratch coat over the course of a day, and loads of materials delivered. This is why I would happily give him an interim payment, subject to knowing he's actually going to do the job we asked him to do.
There has now been a full and frank exchange of views both over email and over the phone. He never provided any square metre measurement on his written quote, which would have avoided this. It simply says 'remove and re-render all four elevations excluding rear extension' and I, perhaps naively, thought that was clear enough, considering I had had a very clear conversation with him regarding the difficult to reach part when he came to quote, and made it very clear I wanted that to be included - I literally pointed it out.
After much arguing, and some polite but very firm suggestion that he re-examine his memory of the matter (I would genuinely give him the benefit of the doubt if my memory of this were anything but crystal clear) he has agreed to include the patch of wall as part of the quote if we make the interim payment. He has confirmed this in writing, spelling it all out somewhat more clearly than the original quote did.
Re interim payments, I have made them before, I think it probably depends on the kind of work that's being done. When I had my central heating done in my old flat, the engineer had me pay a supplier directly to buy the rads etc which I thought was fair enough. The problem here is the dispute, which we wanted to resolve first. What we wanted to avoid was paying him enough to make it worth him cutting his losses and buggering off. Unfortunately that might have had the unfortunate side effect of making him panic that he's going to get paid at all - but again - if he thought back to his original conversation with me he might remember I asked him several questions about payment of deposits etc, and was generally very willing - he just never asked. I think he's just a rather disorganised and could do with writing things down a bit more, personally. He took ages to reply to emails, and I waited over two weeks for his written quote (when he knew I was waiting on it for negotiation purposes with our vendor). While this might have been a warning sign, he has a bit of a monopoly on this kind of work in this area (there aren't that many companies working in lime round here) so I didn't exactly have a lot of choice of other builders...
I do suspect that what happened is he's forgotten about that difficult bit. They're a busy company. We just can't really afford to pay for their mistake without going into more debt, which is why we had to be firm too.