Potterton combi boiler, radiators won't heat up?

My partner and I have just renovated a house and it is on the market, the heating system was installed in the last year but the water has been off a while due to the Water board doing repairs externally. We have just put the water back on and fired up the boiler but the radiators will not heat up at all, the flame symbol comes on at the boiler and on at the thermostat but no heat in the radiators. The water is hot, the flame light flashes on and off itnermitently. Gas man reckons we have a leak somewhere and have to start ripping floors up to find it. Is there anything we may have overlooked so as not to do that?