Just as a matter of interest, what do you think is a "reasonable" price for the apt.? If this is determined by the mortgage you still have, then fair enough.
If you can cover the outstanding balance of the mortgage by selling it, this would seem to be the least painful option.
Of course, this depends on how much of a deposit you had when you bought it.
So, if negative equity is not an issue, I would seriously consider avoiding the ongoing cashflow problems/hassle with tenants/potential issues with Revenue by selling the property and setting up house with the girlfriend with no worries ...