RenderMonkey does not impose any limitations, so it should work. I'm surprised to hear that it isn't. Does the issue happen when you use more than 8 textures, or when you sample from a texture more than 8 times?
Does it happen if you edit one of the samples to use more than 8 textures? or does it seem specific to the effect that you've created? If it is specific to your effect, please send your effect to email@example.com so that we can investigate further.
Thanks for your reply.
I am sampling each texture only once. I'll have to play around a bit to answer your other querries.
Textures are also appear to be mis-matched when you sample from a texture, but do not refer to that sampling.
uniform sampler2D colourMap;
vec3 map= texture2D(colourMap, ...
//no further referrences to map
This caused "colourMap" to replace the sampler directly above it.