There are around 50 species of Mayfly in Great Britain, of which Centroptilum luteolum is one of the more common species. Associated with rivers and lakes it has most likely been blown or flown up from Eastwood Farm Nature Reserve. This is a male, which are known to have large raised eyes as shown below.
We cannot be 100% sure of the identity of this species as there are several other similar mayflies ( Ref 11- CLICK HERE).
Wildlife Garden 