so who is the best and most reliable leasing company, and are there any obvious downsides ?
Use a lease search site like ContractHireAndLeasing.com they'll do the searching for you.
The only recommendation is to look at the total cost of "ownership", often the "cheap deals" are because there's 6 months upfront for example.
I generally ignore the 6+ deals, and look at 3+23 or 3+35
Edit.. ah, yeah, Matt hits the biggest thing to watch for.. options. Whatever option you go for will be amortized over the term, so if you want a £1k option.. that'll be £1000 divided by 36 months on top.
Look for deals that include options like Nav for free. Our E-class doesn't have Nav as standard, so when a deal came up that included it in the price (saving about £40 a month) we jumped at it.