very rare for cylinder to get airlocks ,if its an indirect cylinder with a coil inside for heating then prob near on impossible.
if you got 2 2 port valves or one 3 port valve that controls heating and hot water then i guess you are on a conventional system
it would prob be an indirect system(vented) with a header tank in loft
i can tell you how much parts cost
3 port vlave about 90 quid
2 port valve (you would usually have 2 of these 45 quid each)
tank 180 quid
then there is copper pipe to replace about 20 quid plus tank fittings which are about 20 quid
work that out and then had his labour