I have two jobs - am employed and 'self employed '. Just phone the your tax office up and tell them that you are self employed as well as employed.
You will contuue to pay tax as normalfrom your emplyed job but yyou will gat a tax return each year, which will you will inlcude details of both jobs but they will only calculate the tax that needs to be paid from your 'self employment' as you have already paid tax on the other.
However all this need to be done within 3 months (if I remember righly) from date of starting your 'self employment'. So you may have to have a think about that one.
If you work form home then you can claim an allownace for electricity , gas etc but only a proportion of the days that you work from home a week.
Also travel expenses if your work is based at home.
My first job only just makes the perosnal allownace so anything above that is easy to calculate at 20 %.
My advice is to put money away each month to meet that yeraly tax bill aswell.
Bascially with the tax return you aill only pay tax on what you have arn't so , so if you do only earn £5 per month from your other job then you will pay tax on that amount accumlated over the year.
If you have alot of expenditure on items etc then perhaps get an account to do the return for you .