The Education centre garden is a good place to start your search as there is a patch of lavender in there. Yesterday I saw a lot of bumble bees, several white butterflies and silver y moths. They look like humming-bird hawk moths because they have the same habit of hovering in front of the flowers in daytime, gathering nectar with their long tongues.
Budleia is always a great attractor for the big Vanessid butterflies such as peacocks, commas and red admirals. Small tortoiseshells are still quite uncommon but you can get lucky. There is a flowering bush in Wray House Garden, near the Kingfisher Hide. I found a brimstone there too.
If you spend a few moments on our new dragonfly-watching platform you will almost certainly see a black tailed skimmer that is holding a territory very close to the woodwork.
While you are there, listen out for a turtle dove that has been calling from the dead trees beyond. It is always worth looking at the doves that perch on the power-lines as they are not always collared doves.