> We have tried bleeding all radiators, with the sound of rushing air,
> how ever nothing comes of it, it lasts for about 5 seconds and that's it.
Even if it was a single radiator that had say built up half with air, expect to take at least 10-15 seconds or longer before water comes out (showing that all the air in that radiator has been expelled).
You have to go around each radiator... in turn (start at the lower ones)
then start again.... as some air in pipes may have shifted / air from another radiator moved around..
When you can bleed no more... you are done.
I would have the pump on / system running, as that will help push water up / air out..
AND free up any air-lock that might have built up.
watch out that water will be hot (and black.. from rust and muck) - so have a rag on the floor / under the bleed valve to catch the driibles / rush of water when it arrives. The radiator getting hotter (from the bottom to the top rising) will warn you when it will happen. Slow down / close the bleed valve more at that point.. to reduce the rush,
Also as others have pointed out, check the header tanks (loft space) that they are full
and if it is a sealed/pressurised system (usually Red and white small tanks with a meter under them) there is an expansion tank....
You need to check if you need to open the valve under whichever one is showing low (out of the green zone) pressure. as you bleed more, you will need to keep re-topping that up. Don't leave it trying to top up continuously, as it has a pressure release which just throws excess water down the drain (and prevents damage to the tank) - plus leaves you with a water bill (like running a tap continuously would) !
Strange that no 'professional' has been able to solve your issue...
unless there is another leak somewhere.. same comment .. where is the professional (by recommendation) ?
Have you tried talking to an estate agent that rents out houses, and see who they use (as they DO have to find and keep people that can do a job properly !)