How far apart do the absorbers go on the rear wall and eventually would I need diffusers on the rear wall either side of the absorbers?Part of the rear wall absorbers will go behind the couch. That's fine. Bass is still going through the couch - trust me.
For the corners, look for something that reaches as low as possible and preferably takes up as little space as possible to minimize interference with speaker/sub positioning. As an example, if you look at our 244 panel vs our Tri Trap - the Tri Trap extends out 17" from the corners. The 244's REAR will hit the wall at 17" and then stick out another 5" from there at 45 degrees. Performance still goes to the Tri Trap though.
With all the help you have given me Bryan I would really love to give you the business and order the absorbers from yourself but I get the feeling it will cost a lot to ship over? The quote I got for 3 curved 4' X 2' diffuser panels with nice matching to room colour fabric was nearly £600 and that company was over here in the UK.