What's up with my oregano?

The leaves of my little oregano plant (country cream variety, variegated leaves) are starting to develop brown patches, as are some of the stems. It was bought from a garden centre and is about 6in high, planted in a trough with basil and dill. The basil has developed a grey mould (which we think we've identified as botrytis) and we wonder whether it has spread to the oregano, or whether it's somthing else entirely. (The dill seems to be thriving, although there are some greenfly on it!)

The soil is fairly to very damp (and is mainly peat-based compost). It is possible we've overwatered it. Does anyone have ideas about what could be causing the brown patches on the oregano?

paddy