Good news.. so I sent KARFA's SBC link to the TA and they have successfully TCP linked the three bookings!
However, they did explain that all they can do in Amadeus is put a note on to seat the others 'near' me, not allocate any seats.
I then gave BA gold line another shot and got through to Manchester again, but a much more helpful gentleman this time. He could see that the bookings have been linked and says that's all ok, he could also see that the seats next to me are free and has sent a request to back office to get the others moved into those seats, which they should get done in the next few hours providing those seats don't get taken in the meantime.
Nearly there by the looks of things, fingers crossed! Thanks again for help all.