Re:Tax
As a Chartered Accountant preparing accounts and tax returns for lots of small businesses, i have read these posts about tax with interest.
It is possible to keep the odd amount from a gig out of the tax net if it could be described as "financing a hobby" but there is a grey area between that and being in business as a musician.
I agree that I would recommend keeping a list of receipts, expenses, mileage, telephone costs, etc and showing any profit or loss on a tax return. It sounds like most people manage to show a loss most years - relief for the loss can be claimed on the tax return to ask for a tax refund.
I normally suggest that there ought to be a better year once in 5 years and actually pay some tax, since loss relief is only given if there is an intention to make a profit.
If there is a business, there is a possible liabilty to the self-employed NI contributions, but you can apply for an exemption if you expect your profit to be below £5075.
It is normal to claim an allowance in the profit calculation for using a home as an office for the business - £3 per week is a good starting point.
Hope that helps
I am a newbie as far as playing an upright bass and don't expect to be able to earn anything so am unlikely to take my own advice!