[Geoff F]

I use a Sigma 180mm, Robin, and it suits me; but it is really a tripod lens and isn't cheap.

A 150mm is a bit more versatile and will double up as a reasonable insect lens as well. The 105 is fine but, as others have mentioned, you do need to get that little bit closer.

I have managed acceptable butterfly photos with my 28-135 at the longer end but even here I find a tripod/monopod helps to capture the finer details.

As an alternative you could try an extension tube, say 25mm, with your 70-200 which should get you around 1 ft closer. I worked like this with my 70-300 but you do lose a bit of light and, once again, a tripod makes all the difference. The price varies between brands but you should get one of the top names for under £100. Auto focus should be OK in good light but whenever possible I prefer manual focus for all macro work.