potential clients who haven't registered self-employed but have been for number of years! help!

Model500

I have been approached with the possibility of doing some work for three different clients who have amazingly been working as self-employed for a number of years, possibly 5+ years not registered with HMRC as they were under the impression they didn't have to as earnings under the tax threshold.

My first query is, what kind of submission needs to be made by the client to HMRC to alert them to this fact?

Secondly, would it be best to first establish what the earnings have been per year before alerting HMRC?

Finally, what kind of fee would you charge for doing the donkey work. £x per each year's return based on a normal self-employed return fee?

I have not been faced with someone who has not registered s/e for such a long time before so any advice appreciated.

Thanks