In reply to All:
Thanks for all the really useful replies, and I've trying them out.
I think Rampikino has hit the nail on the head, it's a psychological thing to some extent, (especially if there are people about!).
I tried setting pace alarms, (don't have a HRM), but an inability to work out that I had to set the low pace and not the high pace had me confused for a while. It worked ok on the flat, but as I do hilly/undulating stuff it's a bit of a distraction. Reading some of the posts on this thread a HRM might be the way to go.
Roadrunners advice about not going over slow downhill works, as I actually seem to use more energy trying to keep artificially slow. Just need to work on my quads and technique, as I can't seem to get a fast enough cadence on the steep stuff.
Just got back from a 9.5 miler with 1900ft of ascent and a lot of boggy stuff, averaged 11:25 min miles by easing off a bit on the downhills and run/walking the steep bit (Crag Fell from Ennerdale). Felt easier than it usually does, and actually went quicker doing run/walk rather than trying to run the uphill.
Many thanks for all the advice, very much appreciated.