Right, I've got to the bottom of runtime broker and specifically why it was hammering my hard drive and CPU.
Runtime Broker is actually the Time Broker service which manages background tasks that use WinRT (Windows Run Time) - that's the waffle out the way.
Mostly Micrsoft Apps use this service though there is a risk that other programs might use it so it may stop some programs working properly if you switch it off.
Going to settings-privacy-backround apps will show you which apps are authorised to run in the background and so might trigger Runtime Broker. I have switched them all off except Alarm&Clock and Settings.
I did have "Photo" switched on and as I have over 30,000 pictures on my computer it was regularly indexing these in the background (every day at 2pm or after if it was off or interrupted, from my observations).
I have also stopped all the live tiles on the start menu apart from weather (just right click each one and stop it).
Fingers crossed, although runtime broker may still run, it won't be hammering my computer when it does now.
In a time of universal deceit - telling the truth is a revolutionary act. George Orwell
When the debate is lost, slander becomes the tool of the loser. Socrates