Well 2 more bits came together on the project today.
The carbon fibre diffuser arrived from BMW as did a Hawker 12V racing battery. Only 3.8" a 9" so less than half the normal size but same power and a lot lighter too (28lbs Vs 42lbs)
The battery is going to allow me to halve the battery box and hopefully plastic weld it back together, mount the battery fore/aft and leave space for the new RHS back box. I need to visit the auto electrician Thursday to discuss terminals and cable reroute as I need to use different terminals.
The diffuser is a thing of beauty....I thought trimming it would be a nightmare, but an hour with some masking tape, accurate marking up and cutting has replicated the exhaust cut out on the RHS. I'm also happy that it will mate to the prefacelift bumper as they are exactly the same profile down low. The only difference is that the facelift has a feature line that the diffuser is templated along. I have yet to decide if I will build a fibreglass/ CF profile over the tp of the exhaust or leave it open. Will know when test fitted.
Sorry no photos as it's dark, wet and cold out there....but I'll put some up when I can get.