After some good advice from you guys, I've had a play around with the drawing again.
Side panels in the 'live end' have been removed and will be added after the room has been finished. It might even be worth making the side panels portable so that they can be placed when & where needed. This would also free up some wall space to hang guitars etc.
Coat stand removed. This was just there for aesthetics more than anything else.
Corner trap near the door added.
1/4 size corner trap above the storage area added. Storage is important and I really don't want to lose it. I could always panel treat the cupboard doors if they really need it
Cloud ceiling panel added to the 'dead end'. Exact placement position still TBD.
The two side panels in the 'dead end' have been left in but I fear that they may be too narrow (340mm) although it isn't a problem to alter the sizes. The heater would have to be moved a bit.