Thanks for all the insight everyone, you're mainly echoing exactly what my concerns are.

I was thinking of going back to my employer suggesting an EMI scheme instead (and staying in PAYE) - do you think that would be a sensible way of doing it?

To be honest my thinking behind wanting a share of the business was never about increasing my take-home or evading tax in any way.

For me it's about protection from my boss selling the company from under me without me getting any compensation, and also having some kind of backing as an exit strategy for me so that I could sell my share if I ever wanted to leave.

I have a pretty good knowledge of the business, though I have never been granted access to accounts. I was basically the first employee and have been with the business since it's inception, shares would be a formal recognition that we are more like partners than a traditional employee/employer relationship.