Finally got it out.
I've bought a THA275 with the 2.5m cable loom and was going to install it between the glove box and its lower trim, as has been suggested.
No way it will fit in there!
It's almost too big for the glove box itself....
There's nothing to attach it to under the steering wheel (RHD) and maybe unsafe there if it drops and jams brake pedal/steering etc.
It'll either have to go in the glove box wrapped in something to stop it rattling about, or I waste the £50 2.5m cable, and spend another £75 for the 5m cable to run it to the boot.