So I carried on, still unable to get a picture of a Blackcap. I didn't think the new Boardwalk/Reedbed area was really worth going to, with the possibility of Reed Warblers not due for another few weeks, but I went there anyway. Good decision.
While walking through and stopping to admire a Comma, an explosive sound popped burst out from the bushes about 20m away, taking me by surprise. A Cetti's Warbler in Morden Hall Park! It flew between the reeds for about an hour, while I got frustrated trying to get a picture. All I managed was several poor record shots, but it was enough to prove I wasn't lying!
On the way back, a Mallard brood dozed on a log and I finally got a picture of an Orange Tip butterfly!