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I would be turning up at your property with a pump.
Can you tell me why you think the pump is working fine. is the spindle spinning at a good speed when you take out the centre screw. Also can you turn it up to number three and see if that improves the situation.
I cant see that lead pipe will have any effect but it may be getting blocked. A pump will sound as if its running but quite possibly the impellor is not turning.
I will opt out for now and maybe someone else will be able to come up with a better solution for you.
It definitely sounds like a circulation problem, if you are sure the pump is fine, then it must be down to water quality.
Time to get your system flushed to cure this.
Was it blocked