ICTA1988 s381, how do I submit a claim?

I have a client who has income from employment, but also has sizeable losses from self employment - rental properties. I believe he is entitled to offset these losses against other income (his property losses have been incurred inside the first 4 years of trading) under s381 of ICTA1988.

However, what is the best way to submit this claim, is it to just write to HMRC setting out the facts and the relevant sections of ICTA and their own BIM?

Time is obviously as issue with the self assessment deadline looming, I'm thinking I should file his return as it stands (no extra tax to pay, so no issue there) and then submit a claim for a refund later?

Advice please.