2004 Ford Mondeo rear brake pad replacement

According to Haynes manual the self-adjusting piston has to be screwed in to accommodate new pads (as you would expect). It says use an Allen key, which agrees with the picture, but it doesn't give a size. Can somebody advise please? It looks about the size of domestic radiator fitting (~13mm).

Also the manual says disconnect the handbrake cable from the caliper. But as the cable hooks into the lever on the caliper, you would have to pull the cable back, or push the lever forward, or a bit of both, about 15mm, to unhook it. The cable only pulls back about 1mm, as that is the slack, and the lever only goes forward about the same, as the pads hit the disc. Am I missing something? Slackening the handbrake cable would do it, but that's a fairly big job as the adjuster is above the silencer heatshield and not at all easy to get at.

Grateful for any help.

Roger