Can my employer find out if I work on the side if I dont declare it to them?.?

Hi, I've been working on the side of my main job for about a year. My main employer has just sent a questionaire to me asking if to declare if I am self employed or employed on the side. I'm meant to ask for permission to work on the side, but havent and I am worried that I'm going to get found out. Can my employer found out if I dont declare the work?, through my p60, tax code or anything?.

I work where I get it paid in to my bank via PAYE.

No, your employer will not find out from HMRC, especially if your code number is at Basic Rate (BR) at your second job. If your other earnings are "coded out" meaning your main code number is restricted, employer may wonder why, but Revenue won't tell him, and it's up to you if you do.
You will get a form P60 at each job. An employer only gives a P60 showing your earnings with him, so ignore any answers telling you otherwise.
I think your main employer's concern is to do with conflict of interest, rather than your tax situation. That depends on employer's line of business, and what you are doing on the side. I had a clause like this in my contract of employment, but I never had a second job. Employer could find out if you advertise, or do something that brings you before the general public, like in a bar, shop, or DJ-ing. Think about it carefully, it's your conscience. Consequences could be bad if employer does find out. How important to you is your main job?

Yes, they'll find out when you get your end of year tax summary. If you're getting paid into your bank account for your job on the side then you're getting taxed on it. It will show your total earnings for the financial year - you company know how much they're paying you so if they see a bigger figure on your tax return they're going to get suspicious and start asking questions.
If I were you I would go and speak to someone in your company asap. Explain the situation - if you're working on the side because you aren't earning enough in your current job then they will probably be sympathetic and try and help you.

well, when you do work on the side you usually dont pay tax on it, hence, "on the side" so i cant see how he could have found out. Unless the job requires working with different clients and it spreading by word of mouth

They'll find out with your end of year tax summary,(P60), it will state the amount you have earned, that is if you pay tax on your second job, if you don't then it won't show. Seems unfair though that an employer should virtually "own" you. If you need to top up your earnings then why should you have to tell an employer, you're not a slave, thought that had been abolished.

Hope this helps and have a good day