Hi Dave,
Such frequent activations points strongly to a leak somewhere.
Other than drips from the pipework, have you also checked the water heater. If its a Truma there is a pressure relief valve that goes through the floor, if it is a Cascade then it will be on the outside wall.
It may also be the non return valve associated with the Pressure switch, If this isn't locking off properly than it may be allowing some water back from the high pressure side back into the water supply pipe.
Causes of this type of problem may include a water filter that has broken down and allowed carbon granules into the water pipes, and bits may have become embedded in the NRV seals.
As you have already checked your pipework, then it suggests that the lake may be at teh pressure switch which usually also contains a non-return valve in the cold water pipe. If this isn't sealing properly then it will allow a small amount of water back into the supply line, which you wont see because its still in the pipe.
Other things to check, may have a leak through its pressure relief valve, this will the water heater has