Rear brake rewind screw
 

Putting new brake pads in and winding the allen screw out to retract the piston. It worked on the left caliper but seized on the right one. When I forced it it stripped something (I think a little toothed wheel a the bottom of the hole - hard to see). Anyway piston wasn't looking great so I took to local motor factors and PX for a rebuilt caliper. Only problem is the new one did exactly the same. I've taken it back and will get it replaced for free.
My question is:
Am I doing something wrong?
Are these adjusters made of cheese and a well documented problem?
Or am I just having bad luck?!

Yes the adjusters are made of cheese. I stripped a few. Don't force it.

The adjustment hex has straight splines can sometimes come out and bind, or you may be at the end of the adjusmtent.
Try backing it on a bench where you can see it and know that it is in properly

??

On mine, they turn easily. When I feel it resisting, I stop and crank it back in a quarter turn. They are small components, so it wouldn't take much to strip if forced.

It felt like I was at the end of the adjustment but I still couldn't get it open enough to get over the brake disc with new pads and disc. I guess when they rebuild them they probably just clean them up and replace the piston and seals so might have already had dodgy adjuster.

cheese indeed. If you put a good fitting allen key in and put slight sideways pressure on the key while pulling it out in a downward direction, the key will grip the stripped piece and pull it right out. It's a little gear looking gizmo made of hardened sour milk. The "teeth" on the gear will fail if you look at them wrong. Once you've adjusted the caliper so that it wont back out anymore, you may attempt to compress the piston with *careful* pressure and you might get a couple more millimeters out of it. If you attempt to compress the caliper the way my dad taught me: "Put a C-Clamp on there and crank the shit out of it" you'll likely damage the caliper, but I'm pretty sure you are already aware of that.

Unfortunately, even if you pull out the allen key gear gizmo made of used french fry grease, there's not much you can really do with it. You can't buy a "cog of swiss cheese" from NAPA.

yeah, I've swapped a number of stripped ones out after too much force was put on them. It's just another thing to take out and lubricate along with your slider pins. With a little sideways pressure on the allen wrench as described above, it should slip right out.

I had to soak mine in CRC / WD40 for awhile it was so gummed up as the cover bolt was left off at some point in it's history...but it eventually came out.