By chance I happened to notice a new STOR (Short-term Operational Reserve) site next to an existing substation near Wrexham. Thanks to Russ this is now mapped on OSM at Cefn Mawr.
It appears there are quite a few of these, in various guises, dotted around the country. The idea is that they kick in to manage peaks in demand at 4 hours notice. Capacity seems to range from 3-20 MW and consist of modular diesel, gas or similar generators housed in acoustic profile containers. Presumably potential capacity can be changed fairly rapidly by installing additional containers.
National Grid runs rounds of tenders for these on a regular basis.
The reason I raise it here is that the location of such standby plants may eventually need to be reflected in some solar energy modelling.
I plan to provide some more detailed documentation on the OSM wiki.