The caterpillars (or larva of a bug) are black or mottly brown on the common figwort.It is a quashy bug and emits a juice when brushed up against or is handled. I thought they were rotund leeches when I first saw them but they produce residue like a caterpillar when inadvertently squashed. The eggs appear to have been laid on the figwort and it is dying off where they have been. I rang 18/06/10 and thank you for your help. I attach the photographs.
I am unable with my camera to get a better picture - sorry.
I think I know what you have - it's a bit of a long shot, as the photos aren't very clear
Could this be the Figwort weevil, Cionus scrophulariae?
Have you seen any small black beetles with a cream coloured pronotum hanging around?
The way you describe the larvae just about fits with this weevil's larvae. They are leech-shaped (as you described) and are covered in a shiny sort of slime which gives them this blackish colour - this of course is an excellent deterrent to predators.
To find out more, have a look at the RHS website (especially for instructions on how to control them if they are killing off your figwort)