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It may have been that this was the trigger for some underlying issue that was simply waiting to appear...? EDIT
Am having trouble tracking down exactly which element is causing the fault.
SSL certificate was installed by the server host company - everything appeared to be functioning correctly after this though.
Zend full page cache now disabled during testing.
Site is effectively dead - as customers get bored of waiting and leave!
Adding a product to the cart shows the AJAX loader spinning endlessly, but if a user clicks in the background - they find that the product has been added to the cart. The cart hasn't updated on the page, but if a user clicks the cart - or changes pages the cart updates with the item in there.
Prior to the changes, everything was working fine.
Anyone able to give a couple of suggestions as to what the cause might be \ how to track it down \ similar issues & how they fixed it.