Upstairs radiators keep going cold ?

I have to keep bleeding the upstairs radiators. I have had an engineer round who said it was (obviously) air in the system which he said was hard to get rid off. He also checked the feed pipe/ water thing in the loft, balanced the system and checked a couple of other things. He told me to keep bleeding them and hopefully it would improve, but it has not.

Does anyone know why this is happening and how to fix it and is it really expensive? My house was built in 1982 and still has the original system. I would be very grateful for any relevant information. Thank you.

Sharon Schofield, Devon
January 2009