Just to update the thread, this question is now solved! Courtesy of the following link to a compatibility chart:
http://www.heatweb.com/pdf/Invensys/Programmer%20Compatibility%20Chary.pdf
So after a bit of research I went to B&Q and bought a Drayton Tempus 7 £49.99 and followed the wiring...
The clock dial on the timer has stopped turning properly, loses about 6 hours per day so I need to replace. I can buy a new Danfoss SET3M unit for £43 ish from ebay but i'd rather upgrade to a 7 day digital timer if possible. I've discovered that new units fit to a new standard backplate that...