cold water supply - airlock?

I recently had an ensuite installed which consists of a power shower, sink and loo. The hot water for the shower comes from the main hot wter tank, whilst the cold for all 3 comes from the cold water tank in the loft.
Recently there has been no cold water supply i.e. the shower "struggles", no water out of the tap and the cistern fails to fill. I have managd to get a tube with a cloth round the end for a seal to blow through the pipe in the cold water tank feeding these, which forces water through to e..g the tap and cistern ok suggesting no blockage but then the situation reverts to no water.
any ideas gratefully received as to what might be causing this and any solutions - the recent plumbing has been done with push fit connections
thanks Cheers Chris
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the tap fed from the tank and open both taps. The mains tap is at a higher pressure than the tank fed pipe. This blows any air back to the tank and fills the pipe with water. It is usual to hear the most amazing gurgling. It may also be a good idea to lower the level in the tank to prevent overflow and have an assistant standing by in the loft or the mains tap, just in case of an overflow starting.
MrCheerful
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If that answer fails. This like shows how i fixed a simular problem http://www.google.com/groups?hl=en&lr=&ie=UTF-8&oe=UTF-8&safe=off&threadm=PNbkb.1567%24iT3.12042286%40news-text.cableinet.net&rnum=4&prev=/groups%3Fq%3Dno%2Bhot%2Bwater%2Bwife%2Bgroup:uk.d-i-y%2Bgroup:uk.d-i-y%2Bgroup:uk.d-i-y%26hl%3Den%26lr%3D%26ie%3DUTF-8%26oe%3DUTF-8%26group%3Duk.d-i-y%26safe%3Doff%26selm%3DPNbkb.1567%2524iT3.12042286%2540news-text.cableinet.net%26rnum%3D4
Andrew
MrCheerful wrote:

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