Im guessing with my particular boiler, I just connect the two terminals on the below diagram across terminals 1 and 3, cos the boiler doesn't require a compensating resistor?
That's correct. Wired as SD recommend suggests that the thermostat does not have 240v at the terminals used, possibly only 24V.
SD do recommend that you use an electronic thermostat as it is more accurate. The Honeywell DT200 would be suitable, costs £20-30 depending on supplier/ebay. They use AA batteries to power the display.
Which time control do you have on the central heating?