the red extension button to press is not directly connected to the thermostat, when working there is a fraction of clearance between it and the stats button and this is not enough movement on pressing to give any 'feel' to it having reset or not!
On the other hand, when it has tripped and the clearance removed, you will feel a very small but nevertheless distinct click as it resets.
For the record, exactly the same applies to the Mk1 fanmaster reset which may well also be installed