Being flexible on the car tends to give the best lease deals. The cheapest ones are where a manufacturer is trying to move a particular model quickly.
Most of the lease companies are middlemen, basically putting a finance agreement in place between you and a finance company.
I have just taken out a new lease with Gateway2Lease they have been very good to deal with. Fast communication and for the car I wanted the cheapest prices. Happily recommend them
I use ALD for a car I pay for with an allowance from work. ALD are the finance company, so are good at offering non-standard packages, but lack a bit in the customer service side of the business. I bought a ex demo Audi A5 with their finance, it worked out around £100 per month cheaper than any deal I cold have got on a new one.
Start looking here :http://www.contracthireandleasing.com That searches lots of the deals.
There is a very long thread on PistonHeeads, but there is a lot to filter to find the best deals.