tax man is getting what he is due surely
No particular expertise in this but my understanding is that as Self Employed you are able to offset more "normal" expenditure against tax than you can as an employee. The tax man certainly took an interest in this when I was working as a flying instructor (self employed!) some time ago.
From DirectGov website:-
Whether you're employed or self-employed depends on the terms and conditions of your work. It's important to know your employment status because it affects employment and benefit rights, and how you pay tax and National Insurance.
Basic checks to help you decide
You can usually work out your employment status by asking a few straightforward questions.
You are probably self-employed if you:
run your own business and take responsibility for its success or failure
have several customers at the same time
can decide how, when and where you do your work
are free to hire other people to do the work for you or help you at your own expense
provide the main items of equipment to do your work
You are probably employed if you:
have to do the work yourself
work for one person at a time, who is in charge of what you do and takes on the risks of the business
can be told how, when and where you do your work
have to work a set amount of hours
are paid a regular amount according to the hours you work, and get paid for working overtime - even if you do casual or part-time work, you can still be employed
You can also be employed and self-employed at the same time, perhaps by working for an employer during the day and running your own business in the evenings. Think about each contract separately - you may find that you are self-employed for one but employed for another.
If you're unsure, you should ask to talk to a member of the Employment Support Team at your Tax Office. There's no legal definition of employment or self-employment, so if there's a doubt about someone's status the decision is made by referring to previous judgments - known as 'case law'.
Whether you are employed of self-employed depends upon the facts of your working arrangements, what your contract says or a combination of both.