Following a tip off from a local gamekeeper I managed to find a Kestrel nest in a cavity in an old tree which contained 4 large chicks. One of the benefits of ringing chicks when they are large is that their plumage is more developed allowing many to be sexed.
Reading the “Identification Guide to European Non-Passerines” by Kevin Baker it says, of juvenile and 1st winter Kestrels -
“Some juveniles cannot be sexed with certainty early on but all should be sexable by December having acquired some adult feathers. However, at any age, those with grey heads or grey rumps, uppertail-coverts and tail are males”.
That would lead me to think that this bird is a male and the other three female. It was lighter too weighing only 190g whilst the others were 205-225g.