Currently being shafted by mortgage company

Hi, my fixed rate mortgage ended in April. My mortgage increased by 50%. I couldn't change provider because I am in negative equity (112k mortgage and flat is worth 105). I would love to sell but just not possible. But even to rent out I am making a loss (because of the market rate). If my mortgage provider had not shafted me I could rent it at a reasonable rate and still cover the running costs of keeping it. Which isn't ideal but at least it's workable.

I want my mortgage at a reasonable level. Ok, I know they have me over a barrel so an increase could be expected but 50%! Does anyone have a similar experience or advice how to get it reduced?

Thanks, John