It makes no difference to the tax man whether it's a new van or a used one, it is new to you, so, You can choose to offset 25% or 40% of the purchase price in the tax year you bought it(providing that you are using it 100% for business). After the first year you can offset 25% of it's written back value.
eg: Van cost £5,000. 1st year 40% = £2000 tax relief
2nd year Van value £3,000(or whatever) 25% = £750 tax relief
3rd year Van value £2,250(or whatever) 25% = £562.50 tax relief
4th year Van value £1,687.50(or whatever) 25% = £421.87 tax relief
5th year Van value £1,265.63(or whatever) 25% = £316.40 tax relief
Total tax relief = £4,050.77
eg: Van cost £5,000. 1st year 25% = £1,250 tax relief
2nd year Van value £3,750 25% = £937.50 tax relief
3rd year Van value £2,812.50 25% = £703.12 tax relief
4th year Van value £2,109.38 25% = £527.34 tax relief
5th year Van value 1,582.04 25% = £395.51 tax relief
Total tax relief = £3,813.47
Even at those estimates the difference is only £40 a year tax relief.
It really depends on your financial situation at the end of the first year of purchase, your choice