Hi Tim, I'm registered for self assessment too, but I'm also registered as a self employed smallholder (i.e. the business category is farming).
Income has to be declared if it is not 'incidental' income. So, if you keep a few hens as a hobby and sell the eggs to friends, that is incidental. If you keep a hens for the purpose of producing eggs to sell and make a profit, that's a business (the intention matters as much as the sums involved).
As for expenses, if they are incurred in support of the trade, they get included in the accounts. That means that in our case at least sheep shears count 100%, a wheelbarrow used half for mucking out and half for the garden would be claimed at 50% of its purchase price, whilst a new garden spade wouldn't go through the books at all.
It's pretty straightforward once you know what's needed, but some parts are counter-intuitive, especially around things like capital allowances for plant and equipment. Do you have an accountant? A good one will be worth their fee. Talking of accountants, have you anything to add [member=26320]doganjo[/member] ?