Just had some problems with my Central Heating this week.
But while resolving the problem i found a handy tip?
This isnt for a Combie system, so i'm not sure
if it would work.
Anyway, if you have a Gravity Hot Warter system with a Cylinder
and you think you have a Air Lock?
Use the Cold Water to Push up via the Hot water Taps?
There is more pressure in the Cold you see.
If you have 2 Taps.
Connect with a piece of hose Cold to Hot.
And them open hot and then the cold and the cold will
push the water back through the hot water pipes
Turn the Mains Off (Cold water).
And do the same trick but with your washing machine connections,
just take one off and make a hot / cold loop.
Turn the water mains back on and open the valves.
In the end this wasn't my problem, but a neat trick
nether the less.