If someone who has been unemployed during the current tax year, and not claiming any benefit, does period of 8 to 10 weeks agency work do they need to complete a self-
assessment form now during January for HMRC?
If your total income during the year is less than the personal tax allowance - you won't owe any tax and can't be penalised for not submitting a tax return, unless you get sent one.
If you have other sources of income then you may owe tax on this money depending on amounts and sources.
The time to tell HMRC is after the end of the tax year. The tax returns required now are for the year ended 5/4/10. A letter may be sufficient - neither you nor they wish to get into the tax return regime for a one-off
If you are working though an agency, then you are an employee of that agency and should be on PAYE, with the agency deducting tax and NI.
But it is the agencies problem that they appear to be operating outside the law, not yours.
I agree with Buildersmate- the agency should be making sure tax and NI are deducted where appropriate. But if you only work for 10 weeks you may well not earn enough to pay tax- did you provide a tax form (P60/P50/P45 - I've forgotten which) when you started?