QR ARN to BKK in J for GBP 1290
I haven't had a chance to check all available dates, but seems wide open for January 2020. I've been waiting for these flights to come under £1300 return.
In the end I decided to book an open jaw ARN to CNX (Chiang Mai) and back from USM (Ko Samui). Hop from USM to BKK is in Bangkok Airways economy, but all on one ticket. Total fare was a bit more expensive at £1500 each, but pleased with the deal and flight times.