Hi Gav, wow, you have received total lack of customer service, the fact is that you booked with Thomsons whether they are using another airline for the flight is irrelevent, your contract is with Thomsons. There is a number for queries and booking an holiday but not sure if this is an Indian call centre too ( 0871 231 4691), I have used it in the past and it was based in UK then, even though I had already booked my holiday I used this number for enquries etc because I knew they would answer with the prospect of a new booking and once you get through ask to speak with the manager.
I would ring jet2 if I was you and see if they can help afterall the facility of booking seats was available at time of booking whether you realised it or not so it must still be available now, I cannot see why you would have to wait 7 days before departure.