The brake limiter adjustment turned out to be quite straightforward. Thanks for the advice given.
It is not the original unit and has been replaced in my tenure so was not seized solid or anyhing.
Although my adjustment was perhaps not quite as accurate as it should been (i.e. doing it with an empty fuel tank) the adjustment I have made makes the braking balance far better. While it is still possible to lock up the back wheels, this is far more progressive/less agressive than before.
The squeaking rear brake(s) are more of a mystery. I had blamed the handbrake ... and indeed I adjusted it which I thought had cured the problem. But I did a fair few miles over the weekend and occassionally the squeak would come back. It's almost as if a pad - and it might be the main brakes not the handbrake - is staying on and rubbing (?). But not consistently. It's not a huge deal but it is annoying. Another look shortly methinks!