That might not help. Holiday Autos is just a booking agent. Once you've paid, you get a voucher for the actual rental company (which might be a big name multinational or a local player). Booking through HA is no more (or less) safe than booking through Auto Europe, Expedia etc in this respect.

As for the empty fuel tank trick - just be thankful they didn't force you to prepurchase a tankful (at a vastly inflated price) and still bring it back empty. This is popular on the Spanish Costas, and while some agencies will allow full-full for rentals of <3 days, plenty of tourists have no need for a whole tank in their weeek's holiday.