I've moved this observation from "Other organisms" to "Invertebrates", where more people will see it and we will hopefully end up with a "Likely ID" - hope that's okay.

I think this is the caterpillar of the sawfly Tenthredo scrophulariae - did you note what plant it was on?

Hi Martin
Thank you for the ID on this - we never considered a sawfly as the caterpillar was so large, but agree that the picturs agree perfectly with our specimen.
He was eating the leaves on a phygelis plant in the garden but we also had specimens on an unknown weed at the back of the house.
Both plants were similar, fleshy leaves with bark-like stems.
Looking at the map I can see that these are not common in our area. We will look out for them again next year.
Thanks once again for your help
Christine