HI Dave,
A huge congrats from me. Now to your problem - this low deposit business misleads many people. I was under the impression that if a low deposit was a low deposit it would have to state that clearly on the booking pages and not hidden away somewhere in the small print of the T&C's. In fact I'm sure the T&C's only mention that
IF your dep is a low one that you are liable for the full dep on cancellation. I don't think they specifically refer to any particular booking and cannot therefore be sure if you are booking a low dep hol or a full dep.
When I booked my holiday (different company to yours), my deposit was £30pp and not classed as a low deposit. I didn't need to pay any more until I paid the balance a couple of weeks ago. However another HT member who booked a week or so after me and paid the same amount of deposit said that they were chasing him for the full deposit despite him not being able to recall seeing that is was a low deposit.When I booked I distinctly remember seeing an offer at that time advertising only £30pp dep if you book now - which doesn't manifest itself as a Low deposit.
The only problem you've got is proving what came up on the screen during your booking process. Do you think it might have said - book today, low deposit only £**** pp. As long as the word 'low' is there, they've got you. Sorry if my reply's a bit long winded but if they're saying it was a low deposit I don't suppose you have proof that it wasn't. What do the payments show on your invoice? Where it says monies received, does it state low dep. However, whatever the case, you will get your money back through insurance. The only problem is you will lose a fair bit in excess.
Good Luck and please keep us updated on the outcome.