Readers' forum: Goods for own use

My experience is mainly in corporate tax but recently I have taken on some sole trader clients and am having to deal with goods taken for own use.

I know that if items are taken from trading stock there is deemed to be an open market value disposal. So if one of my shopkeeper clients takes a tin of beans from the shelves I must bring the market value into the tax computation. But when does something actually become trading stock? If my client buys 60 tins of beans from the wholesaler takes one home and puts the other 59 in the shop do I take a trading deduction for the 60 tins and make a market value adjustment for the one tin? Alternatively do I simply include the cost of only 59 tins as a trading expense?

What evidence is necessary? Should the client separate the business...