Once the boiler is running satisfactorily, turn up the thermostat so that it's at a level higher than the current room temperature and, after a few minutes, check each radiator to see if it's warm. If it's not, then check to see if it has a separate thermostat or valve on it to make sure that its open and the temperature is set high enough. If in doubt, set it to full and this should be enough for the heat to start flowing into the radiator.
Put your hand on the bottom and the top of the radiator. If there is a large difference in heat levels, then it's probable that the radiator needs "bleeding," which is when trapped air in the system needs to be let out. The difference in heat levels is because the air won't heat up to the same levels as the liquid in the system. In order to bleed the radiator, you will need either a small screwdriver or radiator key (depending on the radiator) and a cloth. At the top of the radiator, either on the left or right side there will be a small nut or screw. With cloth in hand, slowly turn the nut or screw counterclockwise and you should start to hear a hissing, which is the air coming out. When that stops, liquid will start to come out of the hole. At this point, tighten the nut or screw back up and clean up any dribbles from the radiator.
Once you're happy that the radiators are all warm, you should check for any leaks that they might have. The common weak spot is where the central heating pipework goes into the radiator. Sometimes the seal can fail, which allows water to leak out. This usually shows itself as a small puddle on the floor, water marks on the radiator or, in the worst cases, rusty metal on the interface between the radiator and the pipework.
It may be that a nut simply needs to be tightened up, but in some cases you may need to drain the central heating system and disconnect the pipework for closer inspection. New seals might be required or, in extreme cases where the radiator has got too rusty, the radiator might need to be replaced.
Now is also a good time to get the boiler serviced by a professional so that you can be sure it's in tip-top condition to get you through the winter months. Often you can save some money if you're not servicing your boiler in the middle of winter when the engineers are at their busiest, so do this as soon as you can. As part of the service, the engineer will check the boiler for any fuel or water leaks as well as check the heat generation parts to make sure it's running efficiently.
Hot Water System
Now, onto the water. Frozen water is larger in volume than liquid water, so if the water becomes frozen in a pipe, it can cause the pipe to fracture and leak. Of course, this is only going to happen at the freezing point of water which is 32 degrees Fahrenheit and below. Ensure that any pipework that is potentially going to be exposed to those temperatures is insulated or lagged to keep the water from turning solid. Outside taps are usually a prime candidate for burst water pipes.
It's also a good idea to check that any stopcocks or isolating valves you may have work properly. This is a fallback in case there are any leaks so that a damaged pipe can be isolated properly in order to reduce the risk of water damaging your property. It also makes replacing the damaged pipe so much easier. Locate any stopcocks or valves and turn clockwise so you can be sure they haven't ceased in place. If they have and they can't be freed up, they should be replaced. This might mean turning off the water to the property from the valve in the street. You should contact your water supplier if in any doubt as to how to do this, keeping in mind there may be different regulations as to who is allowed to do that depending on where you live.