I think you have been given some extremely sound advice on this thread Katie - the sort of invaluable advice which would cost a small fortune if you paid a business adviser. I would therefore suggest that you take that advice, regardless of whether or not your current way of working suits you.
To my mind, having had two businesses but in different areas, there are some things which are pretty universal. One is that you need to charge a sensible amount for the work that you do. If you are too cheap, people think you don't know what you are doing, and don't value your research. Also, the monthly payment seems too insubstantial and messy. It implies that someone who appoints you will continue to pay you £50 per month, which might put people off as they would not know how long the job would take.
I would suggest when you get enquiries for your services, you should sit down with the prospective client, find out what they want, and agree an initial fee which would cover your research up to a certain agreed point. That way the client knows precisely what commitment he/she has made, and can then decide whether to re-employ you to do more research once the initial brief has been completed. We all know that family tree research is not like shelling peas - it's impossible to say how long a particular job is going to take, so at least working this way takes the time constraints off you too.
Having said this, it seems to me that you should have put together a business plan before you started, which would allow for the first six months being spent building your business by contacting prospective clients, making yourself known, offering yourself to do talks etc. You will also need a USP (Unique Selling Point) which would make prospective clients come to you rather than to someone else. You are the only one who can tell what your USP is, but I don't think that charging £50 a month for an unspecified amount of work is working for you.
And finally, I started my first business when I was in my twenties, and spent the first six months sitting in an office watching the rain falling and my bank balance dropping ever lower. Everything I tried as far as advertising failed to bring me in additional clients and I was on the verge of giving up. Then I met someone who, having asked for my business card, turned up on my office the next day and offered me a permanent contract with his firm. That firm then recommended me to others, and suddenly I had a successful business on my hands. Of course not all start-up schemes have happy endings, but it really is often a matter of luck, being in the right place at the right time, and just plugging away, handing out those cards and getting your name known.