[charlieb]

It's a Scarce Bordered Straw Helicoverpa armigera - an interesting one, not only does it change colour and patterning as it moults as you have seen but it is not actually a British species (occasionally a few turn up on the south coast having flown over the channel).

It comes in on tomato plants and geraniums (and inside peppers from personal experience!)

If you want to watch it develop into a moth (it's not guaranteed - it prefers the warmer climate of the Mediterranean!) place it in a tub with about an inch of loose soil and some leaves (tomato, geranium or slices of pepper - unless it's eating the lettuce in which case give it some more!).

When its ready it'll bury itself under the soil and pupate. Place it somewhere warm such as an airing cupboard and keep an eye on it! I haven't been able to find any info about how long it'll take to turn into a moth - so keep checking it - it might be weeks or months...