Thanks Henrik.

I thought this might be the case but thought it would be better to confirm my thoughts before pulling out what's left of my hair...
I had considered changing PIC's after one of your previous posts - I have been looking at 16F1825. What do you think of this as an alternative?
I would value your recommendations on other alternates.

For the time being I will just eliminate all display functions and "fly blind". I should at least be able to get the PID routine working, something yet to be achieved since introducing the interrupts!

Cheers
Barry
VK2XBP