I had a verbal advisiory on my parking brake efficiency the year before last, it passed but like Bigjaps was at the effeciency limit ( think it was 16%).
The parking brake seems to receive little attention compared with the disc brake side of things but never the less needs servicing.
I removed the combined disc/drum assemblies and cleaned out all the accumulated dust from them, cleaned down all the backplates to reveal the original brake shoes which still had the blue paint markings on them.
I then lubricated all the pivot points and roughened up the brake linings which still had adequate thickness.
After refitting the drums I then adjusted the brake shoes according to the service manual.
The difference was immediately apparent when I pressed the brake lever down.
This years MOT produced no mention of parking brake eficiency :)
Cost of servicing 2 hours DIY zero, cost of parts zero not a bad result after 20 years.