One small note of caution. Some warm air heaters (e.g. Johnson & Starley Modairflow) use a proportional room stat not a simple on/off type. If the Jemplair is of this type (I can't find anything on it), installing a programmable stat would interfere with the modulation - it would still work but be just on full or nothing. If the manufacturers still exist, it might be worth phoning them to get their opinion first (or a gas fitter with experience of the model).
-Antony