Ok, so I did the rear brakes last night, fitting new discs and parking brake shoes.
I swore a bit trying the get the shoes re-attached until I realised how it could be done but commented to myself that it seemed to be a relatively uncomplicated design. I adjusted the parking brake after refitting the disc by tweaking the adjuster until I could hear the shoes rubbing.
I refitted the wheels this morning and pulled up on the lever - 4 clicks and it's on... fantastic!! I tried to turn the wheel and found - as expected - that the wheel was locked, that is until I rocked the wheel backwards to discover about a centimeter of rotation until it was immobile again.

I checked the other side to find the same.
Anyway - I thought back to see if there was anything I could have forgotten but no, as I said the design is very simple with the adjuster at the top horns of the shoe and the actuation lever at the bottom. However I can't remember seeing anything that might be a positive stop for the shoes - the top floats around the adjuster and the bottom appears to be the pivot around which the actuation mech rotates. My quandary is that this doesn't really strike me as being substantial enough for the parking brake to be effective - or have I got something horribly wrong??
G