As I promised DorsetLady, this is an update on my post yesterday. I saw one of the GPs in my local practice and we agreed on a plan. Up the dose to 20mg (from 15mg) for a week then take 15mg and 20mg on alternate days. Listen to body and go back for a review in due course. None of these rigid plans...
She's a thoroughly enlightened doc - the plan was agreed as a result of a fairly long discussion of GCA/PMR and current knowledge and research. She didn't mind me bringing up ideas from this forum; used a model of a shoulder joint to explain its intricacies; showed me a book (about back problems) that covered stuff like dealing with relapses and preparing for returning to work). A GP with a library in her surgery has got to be a good thing.
With luck I'll be back to work on Friday (immediate boss told me not to try to work for a couple of days).
Thanks again for all your support