I got the longer Torx bit kit from Euro Car Parts in Harrow this morning, and some other bits from Screwfix. I reckoned that I had every option to get the CPS unscrewed. Tried them all, but without some extensive dismantling, I still can't get enough purchase on the screw.
I think the screw is a bit corroded and needs the driver to be perfectly square.
So definitely given up now.
Best effort was with the hex bits from my DeWalt drill/screwdriver, plus a Dewalt universal joint, a hex to 1/4 inch sq convertor and a 1/4 to 3/8 convertor and a ratchet handle.