I was able to get a couple of credited domestic bookings merged to one future booking last week via the phone (and paid the difference on CC), although there was some confusion with merging Seat Only and Seat + Bag bookings, but eventually they just took the total value across, someone else reviewed it and two days later my booking appeared.
This was before (I think) they implemented their online credit call back form submission for credit booking requests, it was the first time the call centre worker had done the process (I could hear him reading through a manual),and obviously domestic rather than TT, so YMMV.