Play this audio clip
We heard this at Middleton lakes quite recently and can't match it to any of the birds we actually saw there, but whatever it was was completely hidden in the reeds.
I think it's probably a Marsh Frog. It's a non-native species which has been introduced to the UK and can certainly be found in wetlands in SE England like Rainham Marshes and the North Kent Marshes. It looks like it's found its way up to the Midlands.
Take a look at this YouTube VIDEO
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Tony
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Pete
Birding is for everyone no matter how good or bad we are at it,enjoy it while you can