My employer is edgy about my side project

What are their concerns exactly?

It's important to make sure you understand why they are apprehensive of your continued work on the project... (it might be something you hadn't considered and can quickly clear up)

If the project has no real cross over with your day job, then I can only assume they are worried about your performance, commitment to the job long term or simply the effect you may have on your colleagues (real or perceived).

I'd recommend a friendly, off the record chat with your immediate boss (maybe over a coffee/beer) where you can do some discovery and mend some bridges.

If things really have taken a turn for the worse, then I agree with Courtland that a quick chat with a lawyer (and keeping a trail of important points/conversations in writing) would be a sensible next step.

List it on flippa, have someone in your inner circle "buy it off you" to create a public record of it changing hands, then tell your engineering manager you have decided to let the project go. Keep it obviously, keep working on it, win. You gave up your leverage by telling them, now go get it back!

Looks like you're from the UK, and so I'm not sure what the laws are over there. But it can't hurt to have a short consultation with an attorney, probably before you have a direct conversation with your company about things.

Hopefully the fact that your project is completely unrelated to your job will work in your favor.

What do they have to do with what you do on your spare time? Just to clarify.

Are they concerned because it's a side-project, or because of the potentially legal issue of making copyrighted football fixtures available?

I can understand the concerns should an employee of an anti-piracy company be caught out 'pirating' the football fixtures

Updates to all - the company has asked me to shut down the website, for fear of some of the content I have there being related to a client, and therefore not wanting any sort of thing associated with it. So, I have done so, the service is not live anymore. I have many more tricks up my sleeve and this one can just go into hibernation for the time being :)
Thanks everyone

You can't expect them to sanction something of that nature. The Formula 1 also prosecute people who repost their live timing data. They "own" the public data. I'm sure having an employee making money from data that a potential partner of their's owns and may one day want to utilize is not a good look.

I’m going to take a rather harsh stance for the sake of making a point. You owe your employer nothing, continue working on it and put number 1 first. The crux of employment is time for money and nothing else, your employer would love to convince you otherwise, but if the money stopped, you wouldn't continue to go into work.

I can see how they could worry about your performance, but if you're gonna code in the evenings you're going to do it anyway, plus they get the added bonus of you levelling up your skills.

I would definitely speak with them and find out what their concerns are and if they ask you to stop working on leverage it for them to pay you more money to cover the time in your evenings weekends that you're not allowed to work.

Mainly though if you're working for an employer who tries to lock you down like that you're probably better off trying to build up your side project revenue and look for a way out.

I’ve been in the situation before where I was forced into signing a non-compete employment contract by a large organisation, I argued at first but could tell it was just standard practise and if I didn't accept I'd be out on my ear.

Lastly as a tangental aside employers are always trying to put you down and make you think starting your own business is an impossible feat, speaking from experience it's not as hard as you might think, if you're a hard worker and good at what you do you can definitely find more creative ways to make money that don't involve employers.