Hi Tony,
I'm sorry that I wasn't able to reply to your query before your visit today (although I think that I saw you and your friend walking back off the reserve), but I hope that you were successful in your search for a merlin.
Unfortunately, there aren't any really favoured places for this species - you just bump into them if you're lucky - although your tactic of checking gates & gate wings may have paid dividends. Tight balls of starlings swirling against the sky is often an indication of the presence of a merlin, but just spending time watching from one spot can also be productive.
And even if you were unlucky, you had a fantastic day for it!
best wishes
Gordon