Fed up of the thing leaning so much, so decided to brave the dark and cold tonight.
Got the valve block out. Sure enough, I had put RR and FL solenoid lids back to front (RR was on FL and vice versa.)
Reassembled, then re-sprayed the plugs in the footwells, and then dried with hair dryer.
Started up, and left it all to repressurise with door open.
Sat in the car, and it all works, perfectly. Button does what iit's supposed to do and everything!!
Wait and see what happens when I drive it tomorrrow, but for the moment, it seems all good. Additionally, the new stereo works great too. Couldn't get it working with the loom adaptor I bought from Nexxia, but it works fine without. The feedback I was getting from the right door speaker was resolved by recleaning and drying the connector in the driver footwell.
Thanks so much for all help!