Thanks everyone for your comments.
I have now been using the 'stealth gear' extreme jacket and trousers for a couple of months and have found it to be brilliant for my needs (currently cold weather and trekking through undergrowth, or sitting in a hide for hours).
As temperatures are now rising, I have been able to open the vents in the trousers and jacket when needed and also remove the jacket sleeves and remain comfortable.
However, as it gets even warmer, it will be too hot to wear, so have purchased a (second-hand) 'billingham' vest for summer use.
I usually carry all my gear in a Lowepro 300 Vortex and leave it in the car when I get to a venue and just carry required lenses/hide/etc to the subject location.