I have quite a bit of experience with this, Andy.
I don't know if it's likely to be of any help though. I do it for a living. Just one-offs like yours. I just go straight to my Bank Manager. I think having a business registered with HMRC and a track-record makes it a lot easier.
For you: if you have a good relationship with your bank, preferably having borrowed (and paid back) before, start there.
Also, maybe a mortgage broker, preferably someone local who you can talk to.
Mamya's link to self-build finance is a definite route for you too. That's the way that people who are not "in the trade" often go.
Remember that, with this kind of finance, there is always someone who will lend, BUT the rates may be dreadful, and the terms onerous.
As always, try and find others who may have done this. There are very helpful self-build magazines.
It can be laborious, but do your homework.