I was sailing last night in a really light breeze and I was really struggling to flick the top batten in the sail. The only boats I've sailed before with any full length battens have been fully battened and they seem to flick from the bottom up. Am I using an old sail which is causing the issue or is there abtechnique?

There are lots of discussions on the forum about the top batten.
Try a search topic batten.

Here's one discussion

Having had similar problems,
I've dredged the forum and would summarise as:
Make sure mast bend is about right.
Make sure head of main is at black band.
A bit of kicker helps
Cunningham does not help.
The top batten should be 'quite stiff'.
Roll tack the boat properly, with a novice crew practice is needed.

But changing from an old sail to a lightly used one helps a lot too in my limited experience.

It's not unique to Merlins, we had terrible problems on an RS800.
Anyone disagree/got anything major to add?

Sounds like a job for the Rutland Training 2011!

We have all the answers!