FYA partial claim

The tax legislation appears to assume that a business will either want to claim FYA on an asset or WDA. It is entitled to do either. It is apparently possible to split some of the original cost so that FYA is claimed on one part and WDA on the other. But a number of reference materials say that a "claim" must be done in order to achieve this split. Does anybody have any experience of whether a claim is required and if so in what format.
Tim Carr