On my first Spring visit to the Woodland/Heath walk I frequent at this time of year, I was able to watch this female Brimstone laying her eggs on a Buckthorn bush. I was talking to a Butterfly Expert at the time who was able to advise on the subject (Thanks Steve).
The Brimstone lays her eggs singularly on leaves on the tips of branches.
Look at the first image below and you can just see the small bottle shaped egg.
Below a much closer shot of the egg.
Later I was able to track a pair of Brimstone during their preparatory flight chase sequence before mating.
Walking under a canopy of mainly Oak trees in the parkland I could hear the call of a Green Woodpecker very close overhead of my position. This repeated several times while I was trying to locate the bird. At the top of the Oak I was under, I found the male bird and started shooting images as in the first three below.
I then noticed more than one bird running around the branch of the tree.
The very poor quality images below show what was happening.
Note from the image below that one of the birds has its hackle up, indicating a show of force to the other bird. It transpired that this was two males having a territorial dispute.
At this point in the image below you can see that one male is making a hasty retreat.
The other male shown here below is still showing an aggressive stance.
I couldn't relocate the other bird, but you can see from the image below that this male can still see the other.