No water flow to boiler

Recently had the heat exchanger (i.e. the bif cast iron block) in our Suprima 40 spring a large water leak. Drained the system from a tap above the boiler. Water draining was rather slow but eventually stopped. Just had the plumber fit a new heat exchanger/boiler to the boiler but now there is no water flow into the boiler. There is plenty of water in the small header tank on top of the boiler mate III. Blowing up the drain valve (above the boiler in the kitchen) blows air up the return pipe back into the main heat store. Putting a hose pipe on the header/filler tank pipe ( the one under the water that appears to go down into the main heat store) is not possible - no movement at all. Does this indicate that this pipe is blocked and hence no water flows to the boiler? How could this be fixed. My plumber is coming back tomorrow but he does not appear to have much idea as to what the problem is. He thinks its an air lock but I am not sure.

Do I need any pumps active to get the water to the boiler or should it flow down anyway by gravity?

Any advice most appreciated.

Thanks

Mark

On reading some posts on the web I think the problem is a blocked feed pipe from the feed/expansion tank into the main thermal store tank. Consistent with this,

1 On draining the system from the kitchen drain valve on the boiler, the feed expansion tank stayed full even though the mains water into it was off, so no top up.
2 On repairing my boiler no water down to the boiler even though water was in F&E tank
3 On opening the thermal store drain tap i only get a trickle of water indicating the main tank is empty (or largely emtpy)
4 When I blow down a hose attached to the F&E tank pipe that goes to the main tank I cannot blow at all. It is as if it is blocked.

Does all this sound as if the feed pipe from the F&E tank to the main tank is blocked? If so what are the fixes if it cannot be unblocked? I hear that you can back fill the system from a fownstairs radiator or the drain tap on the main tank. However, I guess if this is done alone, you will not have any system for topping up the water in the heat store/boiler if it ever runs low, risking damage to the boiler.

Another fix I have read is the drill a hole in F&E tank and connect a bypass pipe from there and fix into the drain tap of the main tank, is essence, bypassing the blockage. Not sure how easy it is to weld a pipe to the F&E tank? Is is copper. My looks like it is copper painted silver. Is this true.

I would really appreciate any advice on your opinion on if this sounds reasonable.

Thanks

Mark

After several explanation to the plumber on how the system works, I persuaded him to do the bypass, running a pipe from the feed tank to the main heat strore drain tap. That fixed the problem and got the system working again.

Thing is now the hot water is really hot, about 80-90oC by my estimate. Can this be turned down or is this an indication of a dodgy hot water temperature sensor. Mine is a bit limescaled up on the out side due to a slow leak. If the GT153 hot water sensor is broken will this result in the heat store overheating and my boiler working harder/longer?

Thanks

Mark