There are a number of types of lease. Contract hire is a fixed mileage over a fixed term. If you are using it for work beware of the clauses about the condition that you return it in. If its got heavy wear an tear you may be hit for high extra costs when you hand it back. Also beware of going over the contracted mileage as they can charge very heavily for this too.
After being stung a few years ago I now get vans on a Finance Lease where at the end of the lease I sell them and pay off the lease company an agreed fixed sum. Usually I sell them for a few £100 more than the Finance balance and make a few quid.
In terms of tax you can claim van costs back on a simple mileage rate back from the business at 40p per mile for first 10K miles and 25p per mile for rest and this works well if the van is travelling a fair way for business. 50 miles per day would be 250 per week and equate to £385 per annum (based on working 50 weeks) this is simple to account for. This includes all costs fuel, lease, servicing etc
The other alternative is to proportion your business / personal use eg 60/40. in this case 60% of the cost of the vehicle could be charged to the business, fuel, servicing, road tax insurance etc.
Hope that this helps - There are plenty of accountants that will advise over the web or phone your own
http://for a quick chat.