Thanks to ChrisR
A simple way to unblock a cold feed on a vented HW system:
Sometimes the blockage is surprisingly soft, and this will shift it back up into the f/e tank where you can bale it out.
You can make the connector from a flexy tap hose and a 15 - 22mm adapter, and a 22mm elbow, all Speedfit. 
This method can be used to fill a system you can’t otherwise get water into, for instance to bleed upper floor radiators. It is NOT a long term solution however, as there is very little reserve of water to replace any losses, if the feed pipe is blocked.
DO NOT leave the mains supply on. Only open the tap in short bursts, until the system is full, then remove the temporary connection.
If this doesn’t work there is no choice but to cut into the feed where it goes into the system and clear the blockage.