I notice the OP says she has paid the holiday in full 2 weeks ago, I'm presuming that the money she sent Low Cost Holidays included monies due for the flights she'd originally booked too. You'd have thought LCH would have informed her at that time about the flights, also she must surely be due a refund of monies recently paid to LCH for the balance of the flights, if they have indeed been lost because of Custom flights going bust??
I'd persevere in contacting LCH, maybe try their booking line as you may have more success in getting an answer on that, than on their aftersales number, and just ask to transfer your call!
Please let us know how you get on and good luck in sorting this out!