Unfortunately Northern Rock have been pretty inflexible (unreasonable) now that they have been bailed out, and should you have needed to move and rent out, they would likely not be very helpful.

With the mortgage LTV that you have you the rate that you are on is not bad.

Also as you are on the SVR they can put the rate up (and possibly down) when they like. It is not tied to the BOE base rate, hence it is at 4.79%.

Sparky.