Purchase button, in cart page, not sending you to checkout, Chrome/Desktop

[SimonKitchener]

In our checkout the PURCHASE button has suddenly stopped working on Chrome desktop.  Seems fine on other browsers and on mobile.  We do have a bespoke theme but the developers are no longer in contact with us.
Nobody here has manually updated the Theme code.
Yesterday I switched to an older version of our theme and that worked once.  Then reverted to that button not working.  However, it does sometimes work if I click UPDATE ORDER then PURCHASE - but just once.
If anyone has any ideas what may be wrong I'd be most grateful.

Thanks

Simon

www.theroadrat.com

[Bundler-Manuel]

Hello @SimonKitchener have you already cleared the cache cookies and tried updating/re-installing the google chrome browser?

[SimonKitchener]

I have updated my Chrome yes and have seen the same error in incognito browsing.  Also it has been reported by multiple customers and other members of our team have tested it at home.  So cache clear and browser update doesn't seem to be the answer I'm afraid.