In reply to Bob Windsor:
Thanks for all the replies! I just got back from the park; after about an hour's unsuccessful searching I finally found three, sure enough sitting near the top of a big Yew tree! Very unobtrusive birds, I wouldn't have noticed them if I hadn't been looking for them.
@Billhook - you're right about the importance of hearing, it just led me to the flock of long tailed tits which I see every 1-2 weeks; definitely one of my favourite birds! And we have a big population of nuthatches which were being very vocal this morning.