There are so many people in your situation Delsol at the moment, including myself, I have two in negative equity, one has been vacant for going on 4 months and the rental income has had a shortfall of �200 plus per month for the past three years.
Not much I can do, but muddle through it and hope things improve. You will have to change to a Buy to Let mortgage, but don't feel you have to stay with the same lender. The problem is that you may not be able to remortgage at the same value, if the revaluation is now lower.
Buy to Let mortgages are always more expensive than residential ones and moreso, if you only want a repayment mortgage, as opposed to interest only.