Hi, I'm new here and hoping you guys can help/

I've just booked a return trip to Egypt for myself and my mum. We've booked with Red Sea Holidays online, which we've done a number of times in the past.
My mum suffers from osteo-arthritis in her ankles, knees, hips and spine and can't get in and out of the normal seats on the aircraft (she doesn't need a wheelchair). In the past I've rang Red Sea and they've always pre booked us medical seats, so I had no reason to think there would be any issue this time.

I've spent the last 3 days trying to get this arranged. Red Sea are telling me that as the flight is with Thomas Cook, they will not allow Red Sea customers to be pre book until 3-7 days before departure.
My concern is if we have to wait until that close to travelling, if there are no medical seats left, my mum won't be able to travel.

I've done some research on-line and found a Dept of Transport Access to Air Travel for Disabled Persons and Persons with Reduced Mobility - Code of Practice. Its a massive document but it looks to me as though the airline has a legal obligation to provide the necessary arrangements for disabled passengers.

Has anyone else had any similar experience? And if so did you manage to get a resolution?