at that age it quite likely has a lot of sediment and may have a partial or complete blockage (there are lots of other problems it might or might not have)
this can lead to radiators going cold, or to hot water pumping over into the feed & expansion tank, and to gas collecting in the radiators.
if it has got a pump, and if water is now successfully circulating through the boiler, cylinder and radiators, you could start breaking up this sediment by going to a plumbers merchant or DIY store and buying a litre of Sentinel X400 cleaning chemical, it will cost you about £15
this is a very mild cleaner, non-acidic, that is most unlikely to cause any damage, it is the one I usually use (I am not a professional). You will know it is starting to work when the water goes jet black with old sediment that has been loosened and is washing round in the water. There are other more aggresive cleaners that work faster.
but first, try bleeding the radioators like I asked and see what you find. Also, go back into the loft and see if water is coming out of the Vent pipe that hangs over the small (feed and expansion) tank. Look and see if there is mud at the bottom of this tank, if so, bale it out and sponge it clean to avoid any more sediment going down.
Do you know where the drain cock is for your radiator system? You will probably need this to draw the chemical down from the tank.
It would be very useful if you can post some photos of the pipes, valves, pumps, wires around your hot water cylinder, and especially to know how many pipes go into the cylinder, where, and if it has anything printed on it.