Just done mine today... Part was about £30 (bought it ages ago) Fiddly job, but don't take long. Undo ABS unit mounting nuts (3) Unclip brake pipes from bulkhead as far as cylinder head. (3 lots of clips I think) Unplug 2 multiplugs from engine side of ABS unit. Ease ABS unit up and out of mounts, removing rubber bushes and metal covers untill its possible to get a hand & allen key underneath the unit. small mirror and good light now needed. put mirror under ABS unit - you'll see 3 allen screws. Undo them - they're loctited in, so are stiff. Pull off plastic switch unit that forms base of ABS unit. Unplug electrical connector, throw unit in bin. Put in new switch & loctite screws in. replace all you removed. JOB DONE. I had to prop the ABS unit up with a piece of wood to work under it, as the brake pipes spring it back down all the time.
By the book, you remove all brake pipes, remove ABS unit completely, but then have to bleed the unit and the brakes, which can mean a TEST BOOK visit and the stealer costs of that!