Bigbadgeorge- I feel for you!!! We did exactly the same thing in 2007 after taking advice from a independent financial advisor. Bought a house that needed lots of work and used the extra money for home improvements, property prices nose dived and the market stagnated. I have spent many nights led awake thinking of a way out. So here's how we did it:
NRAM will allow you to delink your together morgage so you would pay off the secured part and perhaps some of the unsecured from the proceeds of your sale. You can then continue to pay your unsecured loan back over the original term agreed (ie 20 years!) or over pay (there is a limit but its like thousands a month!) All sounds great but... Upon delinking your morgate they will increase the Intrest by 8/9% ON TOP of the already 4/5%, so ultimately you would be paying ~14% Intrest, making it extortionate.
Desparate to move on into a bigger home/ nicer area we worked hard to get deposit together for a new home, then obtained a unsecured loan to pay off the short fall on the unsecured part of the morgate at around 4% APR over 5 years. The day we handed the keys over I was elated. No longer did I have to worry about being unable to get another morgate deal, interest rates rising ridiculously and being able to do nothing about it and being trapped there forever!
We had an element of luck on our side- quick sale for asking price being one of them. Sparkling credit ratings helped. We both have very good jobs with decent incomes and silly decisions and carelessness whilst in our very early 20's has had a negative impact on were we should be financially- but in a few years we will be straight and can put this all behind us.
Talk to your bank- they had dealt with these situations before and were not phased by our situation. We have a few friends who were in the same position and wiggled out similar to us. Nram are generally helpful as they want you off there books. Just clarify your Intrest rates.
Good luck with it all- it's a minefield but there is a way out, albeit a 5 year plan xxx