Power Showers

I installed a power shower a few years ago directly to the mains and it worked fine - around 7Kw and still a good force shower.
After around 18months it reduced to around half its flow - useless
I fitted another shower - higher KW rating but with very little improvement.
Had the water board out to check things who were totally useless - the bloke had broken his pressure meter.
How can I check the pressure - is there anything else I can do?
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