The following configurations are recommended for monitoring tyre pressures on single axle caravans
Monitoring two caravan wheels

Monitoring single axle caravan tyres
Use Tyre Safety Monitor two-wheel setup. TA22/2 ordering and details here
Monitoring the caravan tyres and spare (three wheels)

Monitoring single axle caravan tyres and spare
Use TA22/2 Tyre safety Monitor plus one additional sensor.
Or use the four-wheel system TD1000AX for tyre pressures up to 60psi or TD1200AX for pressures up to 180psi. A single alarm level can be set for all the tyres being monitored.
Monitoring both the car and caravan tyres (six wheels)

Monitoring single axle caravan and car tyres
Use TA22/6 Tyre Safety Monitor for six wheels. Separate pressure levels can be set for each wheel being monitored.
Or use the six-wheel 180 psi system TD2200AX06
Separate alarms can be set for each axle so the car and caravan can have independent settings. See specification and ordering details here.
what stops the tyre sensor from being stolden and can indivsual sensor be purchased
The system is supplied with security collars that lock the sensors into place.
Replacement sensors are also available
November 2010 I purchsed from you a TA22/2 for our caravan, I am about to change the car to a Nissan Xtrail which does not have tyre pressure sensors as did my Citroen C5.
Will the purchase of 4 additional sensors from you be compatible with the TA22 ?
Yes one of the big advantages of the TA22 system is that you can upgrade it as your requirements change.
The user manual includes information on resetting the system to enable you to register the additional sensors and we are glad to advise by phone or email if you need any help.