help with water pump

Hi all,
I really hope someone sees this soon. I've moved into a new flat in the last two weeks. The hot water delivery would occasionally give a really strong burst but generally, was flowing at a normal rate.
Suddenly, tonight, the hot water flow is a trickle and I can't hear the pump come on. I'm by no means an expert on plumbing, but I'll try to describe what there is. I have my own tanks in the flat and a non-combi boiler. There are three pumps, a Stuart FLD-25 and two Grundfos, Super 4/120W and UPS 50 15x18. My limited understanding is that the Stuart is for the hot water, but that could be very wrong.
There is a "master" switch for the booster pump - what the point of it is I don't know but I tried flicking it off then on - no difference.
Anyone have any idea what could be wrong? Is there any diagnosis that I (a plumbing moron) can try? Lastly, is this going to cost me a fortune?
Any help gratefully received as I'd like to shower tomorrow!
A
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In an earlier contribution to this discussion,

Can't fathom your plumbing system from your description, but its possible that you've got a heat bank which is heated by the boiler, and that the central heating and hot water system get heated from the bank via heat exchangers. That could possibly explain why you've got three pumps: * one to pump water from the boiler to the heat bank * one to circulate water round the radiators * one to boost the hot water pressure to the shower and maybe bath/washbasin
This 3rd pump will be controlled by a flow switch. When you turn on the shower (or possibly a hot tap) a small amount of flow occurs naturally by gravity. This is (or should be) sufficient to trigger the flow switch which then turns on the boost pump.
If your boost pump isn't working, there could be any number of causes, including: * duff pump * duff flow switch * blown fuse * wiring fault
If you are competent to work on mains electrics, you should be able to check each of the above and work out where the problem lies.
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