Not really got a leg to stand on sorry to tell you. The rate is classed as VARIABLE and how variable it appears to be is un-regulated. I am sure you are aware of the Credit crunch, the big banks are feeling it also, so they are now paying more for their money, hence your huge hike.
I know this wont help much but sadly is the way it is.
Check your credit agreement as a last chance saloon, if it does not mention variable next to the APR or in the terms and condiditons then do you recall the initial sales call, if you were told it was fixed then maybe grounds for complaints. However if you signed it as variable then that is a legal binding document you are bound to.
Any q's just ask