Peter,
The large diameter Freelander caps are supposed to be more reliable. I have had several stock Rover caps fail. You don`t need datalogging to spot a duff cap, the coolant will expand and when you shut down the engine put your ear near the expansion bottle and you will hear the gas escaping. Someone on Blatchat posted a neat idea, drill out the stock cap and install a tyre valve, the system can then be easily pressure tested with a foot pump and gauge.
Rob