I have read through all the interesting comments here (and on world photography forum) and I realise that 'in the end' it's down to me to decide.
I think I have narrowed the choice down to Sigma 150mm or Canon 100mm (longer Canon too expensive for me).
My reasons are:-
a) the reach of the Sigma (so as not to disturb subject)
b) Canon quality (to go with my 40d)
c) lense length doesn't increase (which I think is important for extreme
close-up's). I understand it does with Tamron.
I was interested in those who were in a similar situation to me recently-what did you eventually choose & are you happy or would you change?
I believe the Canon has full time manual focus as well which the Sigma doesn't.
I'm in the same position. I'd prefer a longer focal length for slightly greater distance to subject but I'm very much leaning towards the Canon.
I use the Sigma 105 f2.8 macro, it's a cracking lens with excellent IQ. When I bought it I did consider the Canon but did feel it was worth the extra so went for the Sigma. Recently I tested a Sigma 150 f2.8 and thought it was excellent, the longer focal length and full time manual focus both seemed like advantages over the 105. I decided that the differences were not worth the cost of changing, however if I'd been buying my first macro I'd definitely have gone for the 150.