I think I am the only travel agent on this board so I can speak with some authority.
If you have paid a low deposit as you have said, i.e. only £50.00 p.p. when it should have been £130.00 p.p. as per brochure then if you cancel you will have to pay the extra deposit to effect the cancellation, i.e. the extra £80.00 p.p. or whatever it is.
If you booked on the web, then you will have ticked a box stating that you accept the booking conditions; it is impossible not to do that because it won't take the booking otherwise. If you booked through a travel agent then you should have asigned a booking form. If you didn't but you have received the ATOL confirmation and you haven't queried anything, then that denotes acceptance of their booking conditions as well. Either way they have got you.
Speaking from experience, I know they have an enforcement department so if you don't pay up they will take action for breach of contract.
Sorry I cannot give you good news but at least it is from the horse's mouth.