Blockage in central heating system - best way to clear it ??

Hi, just had the plumber round to try and fix my central heating system as the rads are not heating up. The boiler works, provides hot water, pump works, rads bled and valve to open up the CH circuit works. Closing off all the radiators in the circuit except the first one results in the 1st radiator getting warm but the return pipe is not. Plumber reckons that there's a blockage in the return pipe as the hot water should be flowing round but isn't. Trying different combinations of rads on/off hasn't solved the problem. He's suggested a "power flush" but he hasn't got the equipment to do it (I'm also worried that pressuring a system nearly 30 years old is bound to cause problems). Phoned round a few plumbers and they are busy or not answering the phone.

So does anyone have any suggestions as how to clear a blockage? or anything else I can try. Have got warm socks/jumpers on whilst trying to get hold of a plumber !!!!