I've just had a look at the CMA page on Covid related refunds and its a bit woolly:
The CMA notes that statutory deadlines for payment also apply in certain situations. gee, helpful.
Anyhoo, I sent DFDS a snippy email right after I wrote the above post telling them that the delay was unacceptable, purely due to their finance policy, and I wanted the refund in the next 5 days. I'll see what sort of response I get from that and then start pointing them at websites if no joy.
I did look up the section 75 credit card claim this morning and got the impression from somewhere that it could take months, but I can't find where I got that from now.