Thanks for your advice guys. I finally used some De-luxe materials View Glue (code AD61) which Gaugemaster themselves recommended. Glue cost me £6.50 from New Modellers Shop Ltd of Peterborough (cheapest I could find it and excellent postal service from them by the way) and it does exactly as it says on the tin. I pasted it on to the back of the paper and then mounted the backscene directly on to the ply backboard smoothing it out with a slightly damp cloth as I went along. You do get some bubbling even so but they disappear when dry leaving a perfectly flat surface. You can also move the paper a bit whilst the glue is still wet providing you take care not to rip the paper. I am satisfied with the result and they claim the 200ml bottle of glue will stick about 7sq m. I found that I only used a small amount on my backscene which is about 5m x 0.3m.
Picture of result attached
Neil.