Wbx's only grass plant (Poa annua I think) has much powdery mildew on the leaves.
There is only one mildew like this that found on grasses and this example must therefore be Blumeria graminis (formerly Erisyphe graminis). It prefers relatively cool and humid conditions and the recent weather must have been much to its liking. While I am sure that this is quite a common species in Sussex, there appear to be no earlier records for the county on the Sussex Biodiversity Record Centre's database. However, I am sure this is simply because people either have not looked for it, or have overlooked it.
Hopefully the grass will flower later in the spring and I will be able to name it with greater certainty.