(UK tax question) Do I have to declare this income?

I returned from travelling a month ago and have had some temporary admi work since then, tax and NI were deducted via the PAYE system.

I managed to get some freelance work through a translation agency as well. How do I declare this? Will the tax office find out if I don't?

You don't need to decalre this until after 5 April next year as this was only casual freelance work - I don't think HMRC would argue you were self employed. At the end of the tax year, you will add all you income together and calculate whether or not there is any tax due on this freelance work. If you are only doing temp jobs right now, you may not reach the tax threshold of 拢5225.
If you are still working, make sure you get yourself off of emergency code and on to a proper footing so that you don't overpay tax. Its better to owe HMRC than have them owe you.

It is likely that the employer will have to itemise payments to "casual" workers otherwise they may have trouble claiming the money against tax.

You are best to declare it as there are harse penalties if they catch up with you have not paid tax. Phone up your tax office - they are surprisingly helpful

You should declare this on a tax returnj at the end of the year. It's likely that those nice people at Revenue and Customs will get your details from the cpompany you did the freelance work for.

I am certainly not going to tell you to wait for them to catch up with you because that would be wrong wouldn't it?

It depends how payment was received. If you received a cheque or BACS payment your bank statements will show income. The person paying the money (if a company) will also have records of the money being paid to you. Legally you should declare this income to the tax office. I rented my house out and declared this income to the tax office by letter, my tax code through the PAYE systems has been changed and this is how the tax is being claimed. Dependant on your circumstances you may not need to pay tax. Everyone has a certain 'free pay' pay amount. The single persons allowance is somewhere in the region of 拢5200. I expect you were taxed on a week 1/month 1 basis when temping and therefore taxed on your earnings as if you had already received your 拢5200. I would suggest contacting the Inland Revenue at the end of the tax year (April 08) to confirm that you have paid the correct tax, you never know, you may have paid too much and therefore get a refund on the wages paid through PAYE. The extra work?? It really does depend on your morals!! Legally you should declare it, but it is highly doubtful the tax office will find out, unless of course some one grasses you up!