There is one at WWT Martin Mere around 5,000 birds according to the notice boards. If you stand in the car park or in the captive wildfowl area (near the visitors centre) you will see them. Last night around 4pm there was a spectacular mass of them. They all roost in bamboo (I believe) within the captive area.
If you look in the 2005 G.M. Bird Report that should give you a good idea of roosts and numbers involved. Personally I`ve seen Starling `balls` wheeling about at the back of Bryn Marsh at the Wigan Flashes, though admittedly there wasn`t `massive` numbers involved.
There's one on the English side of the Solway just before Gretna. I seem to remember a few years ago seeing a huge flock wheeling about when I was coming down the M6.