Mystery shoppers sign up to Mystery Shopping Agencies and will then be offered jobs in their locality. They are given a brief, fulfil that brief, then provide a report back to the Mystery Shopping Agency. They are then usually assessed for quality: spelling, accuracy, detail and objectivity, and this can be reflected in your earnings.
Payments can vary from £5 to £50 depending on the assignment. You may be paid an additional allowance for any product purchases that you have to make, or you may not. A typical example would be mystery shopping a fast-food outlet. You may get paid £10 for this, but you are expected to pay for the meal yourself. You are therefore looking at £5 cash plus a free meal as your total gains.

The amount of work you get will vary from month to month and greatly depends on where you live and whether you are willing to undertake all assignments offered to you - you may wish to decline lowly paid or unappealing jobs. However, if mystery shopping is your thing and your location is favourable, then it is possible to take on up to 40 jobs a week. This would provide an annual return of roughly £10,000 cash plus an additional £10,000's worth of products and consumables (some of which will appeal to you, some not). But to undertake 40 mystery shops a week is a full-time job, and if engaging full-time you need to consider job satisfaction from 'solo reporting' and also compare to earnings for your existing skillset. If you are just looking to sign up to one or two agencies for some top-up income only, then be aware that you will still need to register with the Inland Revenue and complete a tax return.
Potential Annual Earnings
£10,000 cash plus £10,000's worth of products and consumables - taxable
Tax
If you wish to do Mystery Shopping you need to register with the Inland Revenue within 3 months or you could face a fine of £100. Forms and Guidelines that you will need:
Self Employed people with small earnings Guidance.
To register as Self Employed you will need to either ring the IR (Inland Revenue) on 08459 15 45 15 or download and send back the form CWF1.
You may also be required to pay National Insurance at the rate of £2.10 a week, but only if you earn more than £4635 (tax year 2007-2008) after expenses in any one year through self-employment. This means earning more than £4635 in addition to you other employment. If you think you will earn less than £4635 in the financial year, then you can apply for a “Certificate of Small Earnings Exception” which means you will not have to pay NI (National Insurance) Contributions. The Exception application can be found at the end of this document CF10.
Information on Self Assessment.