HMRC may or may not find out about it; the risks and penalties involved if they do find out make it unwise not to just declare it and pay the tax on it.
Based on the fact you asked the question, I am assuming that you currently pay all your tax through PAYE and don't do a tax return. You would need to register for Self Assessment and complete a return; this is not at all difficult if your tax situation is straightforward, which it sounds like yours is. Then you would owe the tax on the additional money, at whatever applicable rate (which depends on how much you earn in your main job, the rate tables are here: https://www.gov.uk/income-tax-rates/current-rates-and-allowances ).
If it truly is a one off you could simply declare it on your return as other income, but if it is more than that then you would need to look at setting up as Self Employed - there is some good advice on the differences here: http://www.brighton-accountants.com/blog/tax-self-employment-still-employed/ :
Broadly, you are likely to be running a business if you have a regular, organised activity with a profit motive, which continues for at least a few months. If the work is one-off, or very occasional (say, a few times per year), or not very organised, or of very low value (say, under £2,000 per year), then it might qualify as casual income.
If you think it is beyond the definition of casual income then you would also need to pay National Insurance, as described in the previous link, but otherwise the tax treatment would be the same.
