Gravity-fed hot water supply problem

[chacha]

We've started having problems with hot water supply.
Hot water pressure drops and then supply altogether stops after 5-10 minutes of usage.
I have checked header tank in the loft and there is water in there.
Cold water supply to shower is OK (also gravity-fed from the same header tank).
I've tried to remove air locks using a "thumb on kitchen tap" method described elsewhere on this forum but it made no difference.
Any idea what to check before calling in a plumber?
Thanks!

 

[chacha]

Problem solved - It was cause by faulty valve on header tank to hot water cylinder pipe. Took me 2 hours and 2 trips to B&Q to replace it
Old olives are pain to remove!!!!

 

[WabbitPoo]

Only 2?

My record was 4 on one Saturday for a "simple" plumbing job in the bathroom a few years ago!