Thanks for the nice comments guys.
Time for an update:
Here's the dents in the backplate from the hammer (x2!). Think I won't be able to sand out easily so I'll probably need to use bodyfiller and then paint after. The backplate is a little scuffed up, but that'll clean up nicely enough.
Here's a PSU bracket I've made. Not secured in to the rest of the case yet.
It's a little ugly with the screw-heads, but they'll sit hidden under 1cm thick neoprene cushioning. Still undecided on whether to remake the PSU bracket from thick aluminium plate (~1cm thick), and simply cut out the fan hole.
Here's the back of the case from the back
Here's the back of the case - Using countersunk screws inside means I need to have nuts on this side. The I/O bracket is inserted from the back to unclutter the look inside. However, the 1mm aluminium IO&PCI bracket has flaps to attach to the case. One of these extends down at the bottom where the nut is at the bottom of this pic. To get everything sitting flush a 1mm step was cut into the inside of the thick aluminium frame with a dremel milling bit.
I decided to use more countersunk screws for a nice clean finish inside.
IC cooling are kind enough to send out free samples of their Thermal interface material if you agree to test it against your previous paste and send them the results.
So after sorting out the kink in the watercooling loop and testing my previous paste, MX-3, using Intel Burn Test, and using InterBurnTest and MSI Kombustor/Furmark simultaneously (to produce as much heat as possible - around 550W!) I swapped out the TIM and remounted the blocks.