Good morning, there may be a few reasons for the fault.
Firstly the solar flat panels may have a build up of dirt which acts as an insulation and stops the suns rays heating up the water within the panels, using a ladder with some soapy water made up with algae remover, use a bristle brush to scrub the panels, this can often be all that is needed to remove the build up of dirt.
It may also be a case that your system has developed an air lock, resulting in the heat not being absorbed properly in the panel, there is often a bleed valve located at the highest point of the installation (typically the solar panel).
Lastly it could be a faulty thermostat, the reading your getting from the LCD may be incorrect and the solar panel is actually 70 degrees, other than using a temp probe on the solar panel there isnt a easy way to check.
Before calling anyone out, I would first give the panel a good clean and see if you see an improvement, if you would like me to recommend someone to come out please provide your postcode.
Should you need anything clarified please ask.
If the panels are clean It must be a faulty sensor or sensor which has come lose.
I would find a handyman who has a roof ladder if it's just a case of reaching over to insert the sensor, it rather depends how much room there is in the roof to move around without disturbing the panels.
It doesn't sound like the sensor is stopping the pump operating so I would be tempted to leave it until you find the pump not activating as it thinks the panel is colder than the water in the tank.
If you finding out the pump isnt starting as the water in the tank is at 60 degrees (which is the standard), then it may be worth getting someone out sooner, however I would have thought the water tank will drop to say 50, the pump will start and the water will heat back to ~60, as the water pumped around will be higher than 57 if we assume the sensor isnt reading the panel properly.