Read back in this thread and Muttleys info is spot on.
Basically someone has tried a quick fix to a handbrake problem thinking just doing that adjuster as tight as it will go does the job. Not!
Jack up the back of the car properly and safely, chock the front wheels.
Slacken it all off then get under and check the movement of the handbrake mechanism on the calipers. I use a long strong flat blade screwdriver to get in and start gently trying to lever the mechanism back and forth. WD40 or the like will help as will a hammer. Don't go too heavy handed, remember you are manually forcing the piston. I'll guess one has frozen and that will be evident but as you are there you might as well run the maintenance on both.
I'll also guess that you will have a missing return spring so get ready to buy some. I got mine from Dave at Seres when they were a forum sponsor, Mike at Listers at now taken that spot so try him.
Once you are happy that all is moving fairly freely and any missing return springs fitted then get back inside and start tightening up on the internal adjuster. It should now pull both sides fairly evenly. Set it so that full handbrake is on with 3 but no more than 5 clicks.
For me the worst part of this job is that stoopid adjuster nut, can't get a wide swing on it so seems like hundreds of little turns.
Not easy to say which side is stuck - the right side may be stuck 'on', leading to slack in the cable, or the left side might be stuck 'off' pulling the cable back. Not safe at this stage to say if one or the other cable has stretched, all the adjustment has been taken up by the imbalance between the two calipers.
It is relatively easy to test which side is stuck by just jacking up one rear wheel at a time (chock the front wheels and check the chocks after you've jacked it up), spinning the rear wheel and applying the handbrake.
One other thing I have experienced since I first commented on this thread: feel each rear wheel after you've been for a journey and see if one is hotter than the other. If it is, it's likely that the caliper itself is seized and the brake pads are always in contact with the disk. This pretty much means you need a replacement caliper - they can sometimes be refurbished but it's not a quick job.
The handbrake mechanism is horribly complicated, due to the regulations that require it to be completely separate from the footbrake. So you end up with a mechanical connection intruding into the brake piston with a self-adjustment mechanism that can jam. Premium cars have separate calipers for the handbrake, or a litle drum inside the disk. These can go wrong too but are at least easier to work on.