Just had a response approx 48 hours after booking - as I sort of expected, the claim was rejected:
Having carefully reviewed the details of your claim, we are unable to approve your claim as the screenshot sent with alternative quote from booking site www.statravel.co.uk
is an open jaw booking, meaning – originating from one airport and returning into another , which is restricted under the terms and conditions for our Best Price Guarantee program, available here
The fare in question was outward LGW-BKK and return BKK-LHR
It was an economy fare which was only £30 or so cheaper on the alternative site - so not a terrible loss - but nevertheless a disappointing first experience with QR!