yeah you have to tell them. i've been doing unpaid voluntary stuff as well as paid working for myself but they need info on all you do.
i went in to office - after an invite by the 'customer compliance people' - and put my case for the voluntary stuff in a way that they agreed to my charging expenses for it - mainly petrol - against what i earn as i still get some benefits... a ramble i know but the bottom line is tell them - i made a full statement, no forms, the guy even wrote it out, i just signed it and have heard no more.