Hi Guys,
Just thought I'd post this in case anyone's trying to replace one of these sometime in the future.
Finally got round to doing this tonight. I ended up buying a 'Lifestyle LP522" Programmer from B&Q (the only one that actually said it was suitable for use with gravity-fed systems). The programmer came with instructions on how to replace some of the older (I assume) programmers out there and so told me which connections on the Sangammo went where on the new programmer. Just took my time and noted the appropriate wire colours from the sangamo and the connection number on it's back plate before disassembly, and it was a doddle.
Next up is replacing our Honeywell thermostat (which seems to think it's 15 deg C all the time). Looking forward to waking up (fairly) warm tomorrow morning!
Leigh