There was a thread about trailer tyres blowing a little while ago.
While perusing the Maplin catalogue I see that they sell a tyre pressure monitoring kit that checks tyre pressures every four seconds and sounds an alert if any start deflating rapidly or drop below preset levels.
it comes with 4 sensors, but says it can run 8, and so would be ideal for most car/trailer/caravan combos. 100 quid for the base unit with 4 sensors A55JX is the code.