Low water pressure / flow

I have got a problem with low water pressure/flow
It only affects my house (not not a problem with supply). All plumbing in the street is new (recently replaced plastic pipe etc)
i have checked the stop tap in the street - which is on full
The flow rate is so poor that if the bathroom sink tap is on, and the bath is turned on, no water whatsoever flow to the sink.
I have found where the pipe enters the property (in the cellar), however when this tap is swiched on or off it makes no difference whatsoever to the water (i.e it cant be switched on or off by the stop tap within the property) I am sure that this is the incoming water pipe and guess that this could be the problem restricting the flow.
Is it possible for a stop tap to fail in such a way?
The problem is, that someone has half plastered the tap into the wall so that it is not really possible to gain access to remove and replace? Is it possible to strip the tap in situ and rebuild? and tips or advice?
thanks
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First stop is to replace the suspect stop cock. If it is half in the wall then it will have to be chiselled out. If it is old don't bother messing with it, just replace it and do a proper job. In fact it would be better to replace the pipe from the cellar to the new street stop cock and have new blue plastic "lareg borer" pipe all the way in. If you have a lead pipe then I "would" replace it.
The water companies generally will connect up a new plastic mains pipe to their street stop cock for free.
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I have tried to open the tap up, but there appears to be some amount of limescale in the tap - Chiseling out and replacing seems the way forward?The pipe appears to be cast rather than lead? Is this possible?
It doesnt look too bad as I should be able to keep the nuts on the pipe and just swap the tap over? Use something like http://www.screwfix.com/app/sfd/cat/pro.jsp?tsF092&id 440

Will have to check this out.
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IMM wrote:

Thames water won't, I asked them only the other day. Have to fill in complicated "New Development" form and get quote from them. Also they want to inspect/certify all internal installation. Note this is a 100 year old property. I couldn't be asked to do all that so am going to leave short bit of lead pipe from their stopcock and connect where it comes into house under porch.
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