It was not until I picked it up I could see why it was holding its eye shut, and this also explained its poor condition. Look at the picture and look closely under the eye. What you will see is the large body of a Tick. The Kestrel appeared not only in pain, but also upon handling was very thin indeed. First job once back home was removal of the Tick, this was done very quickly, then ensuring the Kestrel was warm and fed. Within 4 hours the Kestrel had perked up so much that it thanked me by grabbing my finger and drawing a good mount of blood from me, clearly this bird was on the mend. It may seem surprising that a tick could down something like a Kestrel. However the location of the tick around the eye was causing this bird a lot of distress, this had impaired its hunting which had meant it began to lose condition.
This Kestrel is now in a large aviary and will be released in a few days. I just want to make sure the Kestrel is back up to its right weight and of course there is nothing else wrong, but at the moment it looks fine and seems to have recovered, meaning of course that the Tick did down the Kestrel.