And, as promised, here's the hanging process...
I have a major problem in that it's an old house and the ceiling is 'lat and plaster' - ie. you can't hang anything from it (and don't even think about trying unless you want to spend the next few weeks cleaning up 100 year old black plaster dust - this is the voice of experience speaking from my previous, even older house where that very thing happened)!
The loft is above the studio and isn't boarded-out, so the genius Bert and I came up with a plan to drill holes in the ceiling, push picture wire through and attach it to wood battens spanning two rafters.
So, eyes screwed into the back of the frames ready for hanging:
I held the traps up where they should be and marked where the corners touched the walls and ceiling. Then drilled holes in the ceiling inset a few inches towards the corner. Then push the picture wire through and into the loft to try to find them. Nice...
I then have the fun of crawling across the beams in 100 years of filth trying desperately not to put my foot through the ceiling! Eventually - Eureka! - find them. Here's the piece of wood with the picture wire wrapped round it...
...and the view from underneath with the wire passed back through. The walls are (reasonably) solid, so I rawlplugged one eye in to pull the bottom end back in.
First one hung!
This is a shot from underneath.
For the side absorbers I used right angled brackets on the back and L-shaped cup hooks in the wall and they just slotted in.
A couple of shots of the finished job.
I decided to put the third 4" panel in the back right-hand rear corner rather than in the centre of the room.
And the end result is a MASSIVE improvement in the sound. Still not stellar by any means - but I can tell you that it was so bad before that I couldn't even bear to listen to anything in there it was that bad. I'd been really lucky with the shape and dimensions of my rooms before and this was the first time I'd experienced a square room and was seriously shocked. Once I get all the gear wired up properly and the window blinds installed I'll be pushing the workstation as far back as possible to try to get my mix spot away from the centre and closer to the 38% position. That's going to make a big difference too I think.
I'll be tapping you all up for advice on the back wall very soon - but I'm ready for my well-deserved beer right now!
Thanks to everyone who's contributed to this topic over the years for their brilliant help, advice and experience. I hope this is useful to some other people too.