Thanks Chris, found a really good site detailing stripping and reassembling the mount, including adjusting the worm gear.
http://www.astro-baby.com/heq5-rebuild/heq5-d2.htm
The problem is one of mine is very tight and the other very loose, so which is correct, or is it neither. Running with the cover off it seems the Dec drive is making noise during slew, it is very tight, so I will loosen it up first. Stay tuned
Peter