"working over a certain threshold" doesn't come into it. If you're being payed to play music, then you must declare it for tax. Since most ceilidh band musicians either have day jobs or a pension, the chances are your income from that will bring you over the tax threshold, in which case anything you may earn from music will be taxable at your marginal rate. On the other hand you can offset your expenses - including the cost of PAT testing - so you may end up not having to pay any tax on your income from music.