Easy - get him to sell you the documents (that the customers are on). It's a file that he can sell you, and make a receipt out to you.
At the tail end of the day you can claim whatever you like on your year end returns. It's call self assessment where you are assessing your business. But there is always the chance that if you get investigated by the Receiver then they will find things legitimately claimed by you as not accepted by them. They will have the last say. Work clothing is one example.
I'm sure that the Receiver has a business model for most small trades that your business should 'fit' into. If you are starting off then costs are going to be high and returns low. But in time they will see the % of expenses against turnover, capital equipment purchases etc. Any discrepancies will bring up a read flag.
If you have a large capital equipment expense that year they can see that on your return.
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