Absolutely - if the bank decides in favour of David's, your credit card account would be charged for the amount, and you would need to repay them.If the refund is more or less immediate from a chargeback, but then the claim goes in favour of David’s, the bank would then ask me to repay them?
I may yet accept the credit, as I want to give all parties the benefit of the doubt, but I am concerned about the value of the credit voucher.
I suppose if David’s went bust, I could then try and claim via my travel insurance?
The answer to your second question is not so clear. Once you accept a voucher, I think your chance of credit card chargeback are much reduced. For travel insurance, I think most policies cover specific reservations or travel plans, and not vouchers. In addition there should be a specific coverage for "travel agent insolvency" or similar. If you purchased travel insurance, it may be best to reach out to them and get their advice too (in writing). I think you would have a much harder time to collect from a travel insurance policy than from the credit card - insurance companies just don't like to pay claims, and are ready to go to court over it, whereas credit card companies need to comply with more stringent regulations regarding consumer protection.