Exarmy,
You have been had. There is no reason why just because you are self employed that you can't continue with your NVQ.
NVQ Level 3 has to be completed in the workplace. It doesn't matter what or where the workplace is. It doesn't matter if you are full time employed, part time employed, permanently employed, temporarily employed, or even working on an ad hoc basis. The only impact this will have, is the time it will take you to complete it. If you are full time, permanently employed, you will do it quicker than some one who works on an ad hoc basis.
With regard to collecting evidence for inclusion in your portfolio. You need to do comprehensive write ups of the jobs you do, you need to take photo's if possible, make copies of any paperwork such as certificates you produce, any work permits, isolation permits, lock off permits etc etc. Keep things like installation instructions and so on.
Your NVQ Assessor should also see you on a regular basis to advise you on how to complete the portfolio, he should also do some engineering observations with you as well, and he can also have a professional discussion (which he will record) with you to clear up any grey areas.
Ultimately, once you have completed the NVQ you will sign a declaration to say that it is all your own work. If you do this, and it isnt, then you will look pretty stupid when you get offered a job that requires it, and you haven't got a clue.
Cheers..............Howard