I've booked a 5 leg LHR-TRD overnight return, not quite the same one as I listed but joining up to an early December trip to Sofia (in which I'm planning to actually visit Sofia). I used the cheapest OTA on Google Flights which turned out to be Flightcatchers; after ticketing I was able to select seats via the SAS site, including exit rows. I also seem to have a larger baggage allowance than usual. Flights were £160 - it is possible to get a £124 fare on some days but not ones I could do.

Noting also there are some great prices for the T2/T3 Premier Inn early December, I just got £38 for an overnight. I use it a lot anyway, it's on the 105/111 route to/from the Central Bus Station and ideal for T2 and SAS. But at that price it's a real steal - these are flexible in that you can change the check in date up to the day of the visit.