Thanks for the replies - any thoughts on whether to increase the pressures when fully loaded with a caravan on the back? Chris
Definite YES from me ... I put mine up to 41 F 45 R for towing, I've seen the result of towing with under inflated tyres ... not good...

What folks don't realise is the pressure in the tyre is there to support/stiffen the sidewall, which is actually supporting the weight. If the tyre is "soft" the sidewall flexes a great deal every rotation, out at the bottom, in at the top, this leads to a build up of heat within the tyre sidewall and leads to failure.
The weight of the 'van on the towball, (usually around 80Kgs noseweight) and the oscillating movement of the tow hitch whilst moving over bumps in the road, increases the flexing of the tyre at normal pressure, so more support is required by increasing the pressure.