I have had this problem for the last few years.
I have a twenty year old glow worm hide away 60b boiler upstairs with a gravity feed (indirect system) for the hot water of 2 pipes at 28mm on the right side. Bottom of boiler to bottom of cylinder top of boiler to top of cylinder which is mounted above the boiler.
Two pipes at 22 mm on the left side , top of boiler to pump, bottom of boiler return for the central heating making four pipes in all.
The header tank in the loft is full of water and is feeding the boiler.
At the top of the connection to the coil in the cylinder is a vent which goes over the top of the header tank which is not blocked.
The pump is approx 6 feet from the boiler.
The pump has been replaced approx 2 years ago.
I have try'd the pump at different speeds.
The thing is that if the pump is running i get no hot water.
If the pump is not running the hot water is ok
It's as though the action of the pump somehow disrupts the gravity feed to the cylinder.
I think this may be a blocked cold feed. When the pump runs it creates a low pressure in the system, and the water drops below the level of the cylinder. Try attaching a hose to the vent pipe at the cistern and blowing back through the system with mains pressure, compression fittings will allow you to drop from 22mm vent to 15mm attach hose to 15mm with jubilee clip.