Kev, almost all microwave ovens are made in, normally, one of three power levels, I cannot remember the exact numbers but something like 750w, 850w and 1000w.
The problem with them is not the cooking level, which is what the food industry use as guidelins for cooking, bu the start up power draw which is roughly twice the rated figure, so a 1000w cooker will draw 2000w on initial start.
That is where the problem comes in, if you have other high drain appliances on, such as the electric heating, at the same time you will trip the van electrics and probably the pole as well, which may lead to loss of power to other site users, who may be less than happy.
As long as you go for one of the lower power ones and make sure all high drain appliances are off when the micro is in use you should not have any problem.