Temperamental Hot Water Flow, Vented Cylinder

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Having ongoing problems with a lack of hot water flow from both ground and first floor taps - it’s a basic vented cylinder system, cylinder is fed from the cold water tank in an attic; all the cold taps upstairs and down off the mains, the attic cold tank appears to directly supply only the shower.

At the same time as the bathroom was replaced (about ten years ago), a Stuart Turner shower pump was fitted - at some point, the hot tap flow issues became obvious and the system was replumbed so that the pump also supplies the bath hot mixer tap, bathroom basin hot mixer tap and the kitchen sink hot mixer tap. The pipework doesn’t look great to me, but this worked fine for some time. Recently, however, all the taps have intermittently suffered from very poor hot flow - just a trickle, even downstairs. The flow is low enough not to trigger the pump, so when it happens both hot taps and shower are unusable.

I have drained, scrubbed and recovered the cold attic tank - which wasn’t dirty, but had been left uncovered. I cleaned all the taps’ flow straighteners and the filter gauzes in the shower unit (these were all packed with what look like bits of wasp bodies, I wonder if a nest could have gone into the open cold tank and found its way into the hot cylinder). I also replaced the shower pump, which was getting noisy and had started to leak.

This didn’t make any difference - most times the pump starts without a problem, sometimes it doesn’t start at all and sometimes it makes a noise like it’s running dry. I’m assuming that there is something intermittently blocking the hot cylinder outlet. There seems to be no pattern aside from it doesn’t often happen when the hot water cylinder is being heated.

What’s the correct way to check for a blockage in the cylinder? Could it be an airlock?
 
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Should add the valves from the cold tank seem to work fine, with the pump off flow was as expected, cold tank drain seems fine, and I didn’t run a hot tap until the cold tank was full after cleaning it.
Some photos of how it’s “plumbed”:
 

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Last edited:
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I’ve temporarily supported the pipes that were hanging off the pump flexi, so it’s at a better angle for now.
 

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