I've carried on searching Google and found the following which is an illustration of how many people have this problem with Invensys and Drayton

http://www.chrisrand.com/central_hea...rogrammer.html


.. and finally! here's a link which DOES give you a pdf with the instructions.
http://www.worcester-bosch.co.uk/ind...il&con_id=5066
The product name is Worcester but it appears to be exactly the same device as the Drayton. The clue that led me to this was in another newsgroup.

How frustrating does this have to be?

Anyway, hope this helps a few more people than those who have posted here.