Thanks to you both for your advices. it gives me something to experiment with.
Edward

I have used soldering mat. Toolstation or Screwfix. They also have flame proof glue.

Look for something like Kaowool, its a manmade fibre. I'm using two layers of teh 1/16" thick with a sheet of baking foil between on my Fowler (5" boiler, 1/8" gap) you can get it in thicker sheets or just double up. Not expensive you should be able to do it for under a tenner.
If you can't find it then M J Engineering stock it.
http://www.mjeng.co.uk
Blackgates Model Engineering Supplies also do Kaowool in 1mm, 2mm and 1/4" http://www.blackgates.co.uk

Hope this helps.