Well, I take it all back. My faith has been restored. Barclaycard have called me back, as promised, and given me some good news. The guy could not have been more helpful.
They have given me a number to call (Pegasus) and told me to try and amend my flights with them, with like-for-like flights, and they will cover any difference in cost. He mentioned Atlasjet, but when I told him I had tried to contact them, and they don't fly out of Gatwick or into Dalaman (as per my booked flights), he quickly agreed that they weren't an option then.
In the event of us not being able to get similar flights with Pegasus (which I don't want, as I have already booked with Monarch!), we can raise a dispute the day after our flight should have gone, and claim our money back for misrepresentation. We can also claim for other substantial loss (ie difference in cost of flights, plus extras, such as the Travelodge we had booked at Gatwick etc). I asked him if I could carry on and book other flights if Pegasus didn't have the flights we wanted, and he was happy with that.
I know this has yet to happen, and after my fiasco with Barclaycard last night, I'm sure it wont be all plain sailing (or should I say "flying"!) I also didn't take his number, which is a bit of a pain, though I do have a name for him - probably take me till next summer to track him down again!
But maybe there is a God who's going to help us!

(and hopefully he's holidaying in Calis, so sympathetic to our cause - no, not you Scunner!)

We'll see ................