Late reply - but the wisest answers are worth waiting for. (Read - this is gonig to be dull.)
First - only consider buying this property if you expect to, and preferably want to, live there for the foreseeable future.
Do not rush into the decision. Also - do not rush into the decision.
Can you use the equity in the Bradford property as security on next door? That is - keep them both. If the Bradford tenancy is settled and viable why sell? (Unless the tenancy is undeclared to your lenders?)
Do not expect the market value of this property to return to £270k any time soon.
Your sister's situation is quite different to yours. If she needed to sell quickly she would reduce her asking price.
If you do go for it - consider a fixed rate mortgage. Rates are very unlikely to fall soon - quite the reverse.
Be brave.

Edit: Sorry forgot the nudity - *moons screen* - there you go.
( Larry Vest One day at a time, sweet Jesus, FFS., Tue 1 Jun 2010, 12:46, Reply)