ive had success with builders mates method, if it doesn't work and you have a mixer tap somewhere then try this method
with the plam of your hand hold a plastic bag tightly over the end of the tap to form a water tight seal, open the hot tap then open the cold tap, let the cold water rush up the hot pipework, if its working you should hear the water rushing up the pipes or the pipes may start vibrating, do this for about 60 secs then turn off the hot tap and then the cold tap, then remove the plastic bag and open the hot tap, hopefully the water will now flow.