Thanks for the input, that last link was very interesting. There is very
little on the 'net about weather compensation which is what prompted the
question here.

So here is my list of current assumptions :-

1. I need to keep the return temp as low as possible below 57 degrees.

2. A low temp, long burn is more efficient than a high temp short burn

and here's the long shot

3. Weather compensation does the same as turning the temp knob up or down
relative to the outside temp.

If point 3 is correct then why have an outside temp sensor, surely you could
just monitor the return temp to keep it constant thus compensating for
different losses due to different outside temps.

I'm still struggling to fit modulation into this

Regards Jeff