I will put the document up they emailed me with their working outs, I got it wrong it's 3.4 but can get it higher if i get double glazing and new main doors in right now I have old metal frames that are really letting the cold in and badly fitted front and back doors.
...
So thinking of getting the double glazing done in 2 stages first the front windows and door a d then the back window and doors.
...
The company is MCS certificated and has done its calculations according to the prescribed method so hopefully it will all be good.
Although I can see there is an error in the estimated cost of electricity with the ASHP because they have left out the standing charge for electricity which would be about 46p/day on the basis of the price cap figures they are using. The ASHP cost will therefore be
£168 more expensive then they have shown (although still cheaper than gas). These caclulations obviously exclude the effect of your PV.
Your seasonal performance figure (SPF) is as you say 3.4 and is the most useful figure for calculating the relative costs and emissions.
Before you start looking at double glazing I would ask the installers out of interest what SPF to expect if you did replace the windows (the heat loss/SPF calculations will all be in a spreadsheet and should be reasonably easy to recalculate).
You will then be in a position to see how long it will take to recover the outlay. If they reckon (for example) double glazing would bring the SPF up to 3.6 (around the current UK average) that would reduce your annual heating bill by
£133. If the double glazing (say 8 windows and 2 doors) cost £12,000 it would take about
90 years to recover the cost (i.e. never in the lifetime of the windows).
There are other reasons doing double glazing of course (existing windows are knackered, sound insulation, improve the look of the house etc.) but energy performance improvements can often be modest because windows are usually a relatively small part of the surface area through which heat escapes.
In some cases secondary glazing can be a much cheaper option and while not as good as primary double glazing could still halve the heat loss from your windows at 25% to 50% of the cost of double glazing. Might be worth a couple of phone calls.