Depends what you want to see and why you want to see it. I've only been to Stow Maries once to see a shrike, years back when access was freer, and while it was good there wasn't anything I couldn't see anywhere else locally. Apart from the shrike.
If you're after photography opportunities (for owls, etc) I couldn't help you, wouldn't know the best spots. But I'd guess there might be something closer to south London than Stow Maries and I'd also that you'd have been better off going over winter than now.
If you're going to watch the birds, rather than photograph them, all the species you get at SM are likely to be present in almost any spot of decent farmland/coast in Essex. Let us know what you're after and we'd probably be able to point you in the right direction... but again, there might well be somewhere closer than Essex for almost everything!