OK, so I have now given the idea a full 2 hour session. One modification I did make was to add a support at the back to lift the netting above my head. I always did intend to do this, but was wondering what to use as a support. I ended up using an old Manfrotto 784B lightweight tripod, which did the trick nicely.
It has now got me to about 15ft from the feeders, about 20ft from their main perching opportunities. A couple of things that need a bit of work:
1) The "Jack Pike" camo netting that I'm using could really do with being twice as thick as some birds did notice movement inside on occasion.
2) I need to sort a better way of getting the lens out of the netting as I had a Robin land quite close to me but I couldn't manipulate the camera into the right position.
It's not as uncomfortable as it may look. Getting in and out of the chair is a PIA because it is so low, and because of a previous knee injury I have a few problems, but generally I was fine.
I eventually set the tripod up with the legs splayed quite wide, with one leg directly in front of me and the other two under my legs. This brought the camera closer to me, reducing my need to move.
I also had with me a bottle of water, and my iPad for when it got quiet. However I had great fun using the "Bird Guide" app, playing the call to a Robin that came to feed. He was responding, and hopping all around the garden looking for "me"
Next time out I will be moving the hide closer to the tree where the birds perch before feeding. I can't set up the night before as I'm using my front garden - I have no trees out the back and get very little bird activity. I'm still shooting with my 70-200 f2.8 on my 7D.
Steve