I am currently investigating some problems with damp in my house, five years on from a total renovation.  Not all the problems have gone away and I am quite convinced we never tackled all the root causes.

As this is ongoing, I don't have a final solution yet.  However, I wanted to flag a couple of sites which are worth reading if anyone else is having similar issues (and I know mine is not the only Victorian house round here with damp bits!).

When I did the original renovation work, my house wasn't just damp, it was wet through, due to the previous owners leaving it in a state.  My builder Colin Campbell (who lots of you know but has now moved away) did lots of great work to remedy the various problems and it is WAY better than it was.  I had four damp companies come round to give their advice and most recommended chemical damp proofing.  Only one gave any decent explanation of what actually causes damp though, and we implemented some of his recommendations (non-chemical).

Thanks to a recommendation from James Lloyd, I've been talking to the guy who runs this website: http://www.heritage-house.org/managing-damp-in-old-buildings.html.  

He says the main causes of damp are the following (some of which I fixed before but I'm now investigating the rest!):

There is also a good article here giving a good summary of why damp proofing is largely unnecessary and why rising damp doesn't really exist (apparently it's not even a thing outside the UK...).

I have a lot more to learn about this, but I am pretty convinced that the astronomical quotes for chemical damp proofing are largely a scam.  If you are considering having this work done, at least please read this information and think carefully!  It's a lot of money and may well not solve the problem.

I hope this is helpful to someone and thank you again to James for the tip! :)