Thanks for the plethora of advice. I hadn't realised it would be so complex.

I have a few thoughts to throw into the pot. In my own research at Shimano.com, especially the exploded views and parts lists, I got the impression that the Tiagra (and 105) left-hand shifters served for both double and triple chainrings. This was given strength by the fact that when changing from one chainring to the other there is an intermediate stage between the two (I didn't notice similar effects when I was trying out bikes with a Tiagra triple chainring at the time of purchase).

Also, looking at the Shimano parts lists the only difference in rear mechs is the tension arm cage plates and appropriate spring.

Is this correct or am I being a prat and missing something obvious?
Cheers.