Jack / Harold - Thanks for the further thoughts. To answer your questions;
I used the same make and model T-Stat I had before, which like the Furnace is a Lennox (seemed the easiest way to avoid confusion / proprietary wiring colors issues) Not 100% sure about the Lennox choice of wiring colors but again having replaced like with like I simply replicated the connections as they were.
As for changing from heat to cool, it's an older simple T-Stat that has two switches in addition to the temperature setting. One switch flips the fan between 'On' (what I was referring to as manual mode) and 'Auto', the other flips between 'Heat', 'Off' or 'Cool'. If I put the fan switch to ‘On’ the fan blows either hot or cold air through the house according to whether the other switch is in Heat or Cool mode – as you’d expect. If I have put the fan switch to ‘Auto’ and the other to ‘Cool’, the a/c kicks on and off correctly as you alter the desired temperature setting. However, with the fan switch in ‘Auto’ and the other switch to ‘Heat’ the fan does not come on regardless what temperature setting I put it to. The only ‘clue’ that the T-Stat seems to be working is that as I move the temperature up and down I can hear a clicking from the furnace – obviously some signal is getting to it. This behavior is exactly alike for both the old and new T-Stat. Obviously it is possible that I happened to buy a new T-Stat with exactly the same malady as my old broken one – but I wouldn’t put any money on that coincidence.
I’m sure you're both getting as bored with my problem as I am frustrated with it, but if you have any more thoughts and a moment spare to share them I'd still like to tinker with my tool set before finally caving in and calling someone who actually knows what he's doing !