A follow up on this. First of all I superglued the broken valve as suggested by Mr Linear. It appears to have stuck firm, although I have not driven the car since. As the replacement arrived at lunchtime, it is now off the car.
Second, it is a real swine to unclip and remove. My hands are quite big and I couldn't get my thumb pressed down on the top button of the clip. However, it is possible to see it through the gaps in the casting behind the engine. (This is key to the solution.) Once you have the new one you will easily see it reaches the middle branch of the casting. (Be aware that there is another of these clips on the pipework under the left branch as you look at the engine. Make sure your effort is on the correct one.) For best results you need a long handled screwdriver and an assistant to press on the top clip whilst you press in the bottom and pull it off. Then unthread past everything else, thread in the new one and push it on. Push the rubber hose onto the valve, tie tag and you are done. Now go and put Savlon on your skinned knuckles.
So a DIY job? Yes, but be prepared for some cursing and skin loss.
