If you aren't set on it having to be a large water, I think Caistron Lake, near Rothbury, is well worth a look.
Caistron Trout Fishery - Welcome to Caistron Trout Fishery - fishing lake northumberland
It's not big but it has half a dozen boats, the scenery is stunning and the fish are excellent. It's stocked with good rainbows and has some very decent resident brownies.
If you go mid-week, you may well find you have it to yourself- truly peaceful.
Like I say it's not big but it's a real pleasure to fish. I'd be very surprised if you didn't enjoy it.
Oh, and they have 2 mile of the Coquet that you can have a go at, if you'd rather, for £10/day!!