Whilst poking around on the web, i came across Enerphit (PH assessment for conversions)

Our build consists of large (80m2 footprint) extension to an old cottage(60m2 footprint), with all all be built to highest standards of insulation and best practice air tightness, whilst the new has been relatively easy to construct to the exacting low energy standards the old has tested my resolve on many occasion, and we were advised at the start that to try and convert this old build to full PH standards would be very expensive and almost impossible.

However with the discovery of the slightly lesser PH enphit, maybe it would worthwhile? I have downloaded the PHPP demo, but it seems hard work maybe due to demo version. So a few Qs

1 what is involved in the assessment procedure?
2 what is the cost of the complete assessment?
3 is there anyone on the GBF that can do a cut price job?
4 can i do the PHPP? Where can i get the software (free?)

1. As far as I know, the process is the same as for Passivhaus but the numerical limits are different.
2. Dunno, a few £1000 would be my guess.
3. Dunno
4. In principle yes, and it's easy to do some basic work with it, but there's a lot to learn to use it for a certification so in practice you'd need to commit a lot of time and probably a training course or so. PHPP costs money - it's one way they pay for the development and maintenance of the standards. There is a free demo version you can download.

The AECB forum would be a good place to ask, IMHO. Several people there are quite heavily involved.