Interesting info to my original posting. Here's some further info about my loft space.
I have an end terrace house built in 1984
Ventilation is provided at the eaves as well as having an air gap built into the outside wall of the roof space. It's about a metre tall and very narrow. I won't be shooting any arrows from it.
Currently there is some scanky old glass fiber in between the joists. I had originally thought of using Celotex between the rafters to provide extra insulation. I would ensure a suitable air gap between the Celotex and the the felt sarking. The Celotex would stop short of the apex of the roof and the eaves.
I am concerned about ventilation and condensation in the loft. Despite the existing ventilation solutions (eaves & brick air gap), there is significant condensation during winter especially on the north facing roof of the house.
Anyway, I am now thinking of upgrading the insulation between the joists by using Space Blanket from Knauf. It seems a clean and tidy solution which I can lay down on top of the existing stuff. Hopefully it'll reduce the amount of free floating fibers from the existing insulation.
Has anyone any experience of using this stuff?
Thanks.