Hi Ralph,
the window didn't work ! Initially because the 'best' window position only let in afternoon sun which gave too much flare for my lens (yes I could have blocked that direct light into the lens but too much hassle again). The other problem is that with my lens it was impossibly difficult to get diffuse even light because the background wasn't sufficiently out of focus. So I then got a pane of glass from a photo frame and positioned it in a perfect place looking out to some trees, blue sky and a grey wooden fence, only to have the same problem with the background being too in focus. The other problem I then encountered was clouds moving into view changing the exposure levels !

Can you comment on my next plan which is to get a piece of bright white cardboard and use that as a backdrop reflecting direct sunlight through a pane of glass about a few metres away upon which I have taped the transmission step. The white card should be neutral (i.e. have no colour cast) (excluding effects of any optical brighteners fluorescing). I would then set my camera exposure to a grey card sitting in the same sunlight, or even use a hand held light meter to get the incident light level.
regards
Peter