Capital allowances on sole trader van

Hi

New to site so please be gentle.

I have been asked to complete a friends self assessment for the tax year ending April 2011.

He's a bedroom fitter and owns his own van which has had for a number of years. The bookkeeper who filed his previous tax returns did not take into account any capital allowances for his van.

Even though he has not purchashed the van during the tax year we want to start claiming capital allowances against it. Can I start as if it's the first year or even back date any of the allowances?  

Any advice would be gratefully received.

Many thanks

Jonny