One tangential word of advice: be very cautious about telling anyone that you are doing this; your company, when they hear about it indirectly, might feel they own it.
Your contract may or may not make this ownership issue clear. The laws in your country may or may not make this ownership issue clear. However, that's all irrelevant. If anyone at the company feels that the company ought to own your product, then it gets very sticky, uncomfortable and possibly expensive.
And you really don't want to end up without job and side project.
This reply applies to robwgibbons - thanks for the replies guys.
I had started work on this product before I started working at the company - it just happens that the company had a need for a similar product and had chosen (I believe in the year immediately prior to my joining them) to develop their own.
Also I can't remember the wording, but my contact does explicitly state that any of my own work outside of working hours does belong to me. This was added to my contract by my boss after I asked about their policy during my interview. I should also note that the system that the company developed is not (and will never be) for sale or for use by anybody outside of the company.
That said, nobody at the company knows about the specifics of my side project, only that I work on something in my own time.
Your contract may or may not make this ownership issue clear. The laws in your country may or may not make this ownership issue clear. However, that's all irrelevant. If anyone at the company feels that the company ought to own your product, then it gets very sticky, uncomfortable and possibly expensive.
And you really don't want to end up without job and side project.
That's my tangential advice.