Couple of ideas spring to mind:-
- get your folks to act as guarantor on your loan, this should bring the rate right down. Its probably why Natwest offered you such a high rate in the first place as ideally they wanted more security
- have your folks take out a loan on the premise that they will potentially have a better credit standing than you and you make the repayments to them
- your folks could get a further advance on their mortgage which is usually a low interest rate and typically the same as their existing interest rate
There are of course the obvious implications of you being able to repay and whether you (and them) are prepared to take on the risk of your parents losing their home should you default. Lots for you and them to think about.