I'm no expert but try bleeding the radiators? I was always amazed at how much air used to get in mine which would stop them heating up. Soon as I bleed them, they would heat up again. You can get a little key that goes in the valve usually at the top on the side of the radiator. Get a cloth and hold it under the value as you slowly open the valve with the key and let out any trapped air until you start to get water. Do the same on every one in the house. If you do it when the heating is on, the pump will pump air around the system and you should be able to get any air trapped, out. If it is trapped air, you should start to feel the radiator start to warm. If they don't heat up, then it maybe another problem sorry.
Hope that helps and hope you can get your folks home warm again soon and most of all, without spending money!
Edited by MoretonExpat (7th Mar 2017 8:51pm)