Hot Water Cylinder

I have been looking at UK d-i-y forum for the last 3 weeks and have not been able to find an answer to my problem.
When the heating has been on for about an hour or so (when it is at full temperature - 60 degrees), there is a dripping sound in the h/w cylinder and this makes it sound like the cylinder is at least half empty because it echos. The hot water tap upstairs is fine but the rate of water from the kitchen tap which is a mixer seems slower. Now and again the overflow pipe drips from the hot water cylinder header tank. The heating system is a Y plan with an indirect cylinder. At the bottom of the cylinder, the return flow from the indirect part has a gate valve on it which is partly open - am I right in assuming that this is working like a lock shield valve.
The noise is obviously not normal and something is amiss- what is the correct way to check, fill and maintain the h/w cylinder.
Any help would be much appreciated.
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Is the cylinder orientation conventional? I mean does it stand upright with the vent/outlet pipe coming from the top?
Christian.
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