Potterton Profile 50e?

We have a Netaheat Profile 50e that is not heating all of the radiators in the house, of the six, only the three closest to the boiler are heating up. I have a feeling that there is either some sort of air lock or air in the system, or more probable that the system is low on water and is not circulating properly, and/or perhaps the pressure is too low. We had a heating plumber in about six weeks ago and he topped it up and it worked fine, I think this may need to be done again. The problem is, I texted our landlord to say no heating and this is one of the coldest winters in decades here in Ireland and the roads are a mess. I could envisage no expert help coming for at least a day. How difficult is it to top up and re-pressurise a system?