The fact that the hot water is at mains pressure is NOTHING to do with the pressure in the rads, boiler and heatstore. I assume you have the version of the B-M 2 with a header tank on top of thestore itself. That's where all the water in the rads comes from and the pressure will ONLY be determined by the height of the header tank above the rads. You'll immediately realise that (especially in the case of upstairs rads) the pressure will be minimal, so airlocks and rads that won't fill are very likely.
Can't really help from here.
Couple of things to note / try: The heating pump must be OFF when you're trying to fill the rads. If it's on, chances are that one or more airvents of rads will suck instead of blow! Depending on the rad type(s), position of drainoff points, and also what fittings you have available, you COULD try filling the inactive rads with a hose connected to one of the half-inch connections at the top of the rad, or via a drainoff. This will (hopefully) have the effect of pushing the airlock back up through the pipework all the way to the heatstore and into the header tank, where the air will come out as bubbles. WARNING: If you do this, the water level in the header tank will RISE. Make sure it has somewhere to go to - ie. a fully-working overflow pipe to the outside of the building. Also note that the heatstore is NOT designed to take mains pressure. If it accidentally gets pressurised it WILL burst.
To be honest, if you've added rads & pipework, you might be well-advised to get in a professional and stop banging your head against walls! :=) Give me call on 07958972051 and I can give you the number of someone local to you who could help (for money!).
Can't really help from here.
Couple of things to note / try: The heating pump must be OFF when you're trying to fill the rads. If it's on, chances are that one or more airvents of rads will suck instead of blow! Depending on the rad type(s), position of drainoff points, and also what fittings you have available, you COULD try filling the inactive rads with a hose connected to one of the half-inch connections at the top of the rad, or via a drainoff. This will (hopefully) have the effect of pushing the airlock back up through the pipework all the way to the heatstore and into the header tank, where the air will come out as bubbles. WARNING: If you do this, the water level in the header tank will RISE. Make sure it has somewhere to go to - ie. a fully-working overflow pipe to the outside of the building. Also note that the heatstore is NOT designed to take mains pressure. If it accidentally gets pressurised it WILL burst.
To be honest, if you've added rads & pipework, you might be well-advised to get in a professional and stop banging your head against walls! :=) Give me call on 07958972051 and I can give you the number of someone local to you who could help (for money!).