Anyone extending where party walls are involved just HAS to build the cost of party wall surveyors into their budget these days (c.£1k+). It's generally a fact of life these days, esp as people no longer communicate in a way that they might have in the past. If you're building onto their wall/foundations, then bear in mind you'll be liable to pay them for a proportion of the cost of those but your surveyor will advise on that.
I think a verbal conversation would have been the best opening move and, hopefully it's not too late for that. Our neighbours just put letters through the letterbox and expected us to sign things that we weren't completely sure about and they were never around when we were available to discuss it. Consequently we couldn't sign within the required 14 days and ended up with our own surveyor, and I'm glad we did as their works caused damage to our property and it was good we had our own representative to sort things out as I felt they had no consideration for anyone but themselves.
Our other neighbours, on the other hand, knocked the door beforehand, chatted to us about their plans and explained things. We used their surveyor for the party wall survey, which kept their costs down, and it was a much easier and more pleasant experience.
You say your neighbours have been difficult from the outset but in what way? The tone of your post doesn't exactly sound particularly friendly, maybe they don't want to talk to you for that reason. It's not their fault that your budget is tight!
Perhaps your neighbours don't know what to do, so to do nothing is easier. Why not knock the door over the weekend and try to have a chat to them?