Thank you ever so much for these answers. Now that you mentioned CATERPILLAR, I dont feel so guilty for mistakenly squashing the one that fell on my patio from the bunch of garden cuttings I was carrying to my compost bin last night.

It was huge!! And although it was getting dark (I wanted to make good use of the sunny evening after work), I could see that it had hairs and was about an inch long. So, this gives me a clue as to what to look out for. Earlier in the Summer though, I had turned the leaves over to spray them and seend some little black clusters of black dot-like insects.

Hi Gm, blackfly love nasturtiums, that's probably the black dotty things:-)

I direct my cabbage whites to the nasturtiums in the hope it will keep them off the cabbages!

If it's black fly, and from what you described it sounds as though it is, you can use a strong soap solution to get rid of all aphids, washing-up liquid will do fine... Cheap, effective and nature friendly.

Thank you :)