Damaged chimney?

[gavski]

I have a "living flame" type, open gas fire in my old hearth with a flu up the chimney. On top of the chimney is a metal vent.
Whenever it rains, bits of crumbling mortar and dirt fall down into the hearth. Birds also drop stuff down it.
This leads me to believe that the top of the chimney must be open to the elements and probably needs repairing?

I would like some advice on who would be the best person to look at this - a builder, a roofer, or would I need a specialist? and how much should I expect to pay for a repair?
The house is 1950's so It's not a fancy chimney stack.

thanks

[welsh brickie]

you need a gas cowel fitted on your stack,the vent is cheap but the fitting cost will depend on how accessable it is.
A roofer is probably best to fit it.