Earlier today I drained out the cold water tank in the loft to work on a leaking pipe,I turned the stopcock off to prevent the tank refilling and drained the tank by opening both taps in the bathrooom, after I had finished I turned the stopcock back on and waited for the tank to re-fill...now I have only a trickle coming from the cold tap at the bathroom basin...bath is fine and the toilet flushes and re-fills but nothing(much) from the basin tap...could it be airlocked or something?