My flush has started to make a loud noise every time it is pulled. It sounds exactly the same as the noise you hear if you had an air lock in the pipes. If you run a cold tap elsewhere in the bathroom the noise stops. As soon as the flush re-fills however if you pull it again the noise starts again!

There are no other problems in the rest of the house. The gas heating system is only two years old and all the pipes and radiators were new at this time.

Can some one suggest what the problem is?

What kind of toilet?

Just your average normal close coupled toilet!

Does it have a name?

No

Mine is named johnny. OK just fooling with you but I don't know what you mean by "close coupled toilet" and the brand name should be on the toilet just behind the seat himge.

Close coupled simply means a 2 piece, bowl and tank, rather than a 1 piece

If you run a cold tap elsewhere in the bathroom the noise stops.
This nails down the source to the ballcock. You can try to regulate the noise by cutting back on the shut off valve or you could shut the water off and open up the ballcock. Place a can over it and turn the water back on for a moment to flush out the supply and ballcock. Reassemble and let me know. Good luck, Tom