Is it legal for a internet bookie to change odds on a winning bet after you win?

I had placed �17.50 at set odds of 7/2, then when I checked my winnings it said 'palaple error' and the odds should have been 1/7, winning me �2.80! From VC bet, I think based in Gilbratar. Is that legal for them to do that, or can I insist they pay out proper amount?