The bay markings have worn away, as they often do on cobblestones probably faster than usual road surfaces and Bristol has a duty to maintain them.
But there is a parking sign near your car pointing to the machine. The case depends on your duty to check for signage in the vicinity of a machine and probability of unrestricted parking vs their duty to maintain markings.
Maybe they'll roll over and cancel but you'll only find out when you make a challenge.
