Hi mojo
Without proper figures , it's impossible to advise. Interest rates, term to go on mortgage, etc.
You should be comparing the rent with interest paid on the mortgage, not the mortgage repayments which include capital.
If you have a cheap tracker, it's likely that most of your repayments are capital and you will be eliminating the negative equity very soon.
If you do choose to trade down, and if you have a cheap tracker, there is a good chance that the bank will allow you to keep your tracker.
If you have a cheap tracker, and you take out a much smaller mortgage but at SVR, you won't be saving much.