BunnyJ
Thomas Cooks seats arn't to bad, but dont make yourself uncomfertable wearing the belt, if you need an extension ask. I am cabin crew now and have been for 6 years and I cant tell you how many times a day we get asked for them, and crew dont bat an eye lid,we dont run back to the galley and start saying "jesus lard arse in 34F needs a fatty belt" half the time we just give it out, although I have to admit i usually hide it in my hand when handing it over just incase the passenger is a bit embarrased.
Please dont buy your own off ebay as suggested, it may not fit thomas cooks fitted belts, and it may not work in the same way, so god forbit should anything happen your belt might not release, plus i dont think the airline would be to happy about it if the found out.
honestly, i dont think you have anything to worry about! just ask for them belt if you need it, im 14sttone and 36" waist and when i fasten a buckle onboard the buckle is at the end of the strap lol