Being self-employed when you are not at work is quite common - lots of people do it so just check with your employer first.
And check your contract of employment.
Obviously employers would not be happy if you were in competition but if that doesn't apply there shouldn't be a problem and i don't think they could stop you - but I'm not sure.
Then to work for yourself you must have your own GSR which is less than £200.
You should have public liability insurance - possibly around £300.
Then away you go.
But you must keep really good records - for Tax - for 7 years:
all the places you visit to do quotes,
all the jobs,
all expenditure and all income.
Record miles driven - to get materials etc.
Expenditure on stationary used, phone costs.
If you are only doing a small amount of work and keep good records yourself your accountancy bill could be less than £200 per year - but it could be higher.
If you have a van/tools/phone supplied by company you must not use them on your own job (unlesss boss says it is ok!).
And then what ever is left as profit will be taxed at what ever rate applies to you.
And as said by others there may be a small amount of National Insurance.
Don't be scared to phone up HMRC to ask questions about tax - they are mostly helpful and friendly people!