Tax From Website Advertising?

I'm hoping that I can get some advice on the following:

I'm a PhD Student who gets a £12k maintenance grant a year. This is untaxed so I'm fairly sure but not certain this will have no bearing on the following.

I have a website that gets quite a lot of traffic.

A network of websites has offered to bring us into the network and allow us to use their advertising system. This advertising system is run by a third party.

Any ad revenue we make will be split 50/50 between us and the network.

Potentially we could be looking at making between £500-£1000 per month from this. Realistically I think this figure is more likely to be around £100-£200 a month.

This would more or less cover running costs. If we earn a bit more this'll just go back into the site for running competitions or buying hardware, etc so we don't really make any profit on this.

There's also the complication that our website's been running for years but if it only just starts earning any money I'd have to put the start date as now which might confuse things.

As the situation seems a bit complicated I'd rather email/write to someone than try to explain all of this over the phone but I'm having no luck finding an email address at http://www.hmrc.gov.uk/ for what I want. Does anyone have any idea where I can write to to explain or should I just fill in the self-employment form (CWF1?).