I have a mac book air. Have had no issues with any virus before but today my safari page keeps directing me to a page which says " cyber crime" posing to be by Cheshire Police AUthority uk metropolitan police.Tells me that I have 24 hours

You should erase the drive and reinstall, do you make backups?

"but I am unable to click on the reset safari tab as the pop up for stay on page keeps coming up"

The pop ups are due to malware not a virus.

Use the malware removal tool here > The Safe Mac » Adware Removal Guide

This is simply a scam website. You do not need to erase the hard drive over this. Also, as much as I appreciate Carolyn's referral to my Adware Removal Guide, that won't help with this particular issue.

The scam attempts to fool you into believing that your computer is locked, and that it can only be unlocked by "paying" a "fine," thus giving your credit card number to the scammers. When you force-quit Safari, that same page re-loads when you open Safari again, because Safari is trying to be helpful by restoring you to the state it was in when it "crashed." (A force-quit is essentially a crash.) You can prevent Safari from doing this by holding down the shift key while opening Safari.

Then, just be sure not to go back to the site you were on when this page appeared. If you do happen to run across it again, just force-quit Safari and re-open it with the shift key down again.

It's not a virus. It's a JavaScript scam that only affects your web browser, and only temporarily.

Some of those scam pages can be dismissed very easily. Press command-W to close the tab or window. A huge box will pop up. Press the return key and both the box and the page will close. If that doesn't happen, continue.

From the Safari menu bar, select

Safari ▹ Preferences... ▹ Security

and uncheck the box marked Enable JavaScript. Leave the preferences dialog open.

Close the malicious window or tab.

Re-enable JavaScript and close the preferences dialog.

If the Preferences menu item is grayed out, quit Safari. Force quit if necessary. Relaunch it by holding down the shift key and clicking its icon in the Dock. From the menu bar, select

Safari ▹ Preferences... ▹ Privacy ▹ Remove All Website Data

to get rid of any cookies or other data left by the server. Open your Downloads folder and delete anything you don't recognize.