Fluctuating Water Pressure

This is a bit involved but please bear with me.

I live in a first floor maisonette that shares the mains water pipe. A couple of years ago my neighbour downstairs installed a central heating system that meant all hot water went through the new boiler.

Before that (as I understand it) the mains supplied the water tank in my loft and water was heated via an immersion or tank in the flat below. The cold water supply was from that attic tank too. The plumber who did all this said that as the tank was now not being used no water would be entering and what was in there would dry up.

The situation I have now is that I can be doing the washing up or in the shower and the water pressure will drop. In some cases the water stops and I have to turn it off as I'm worried about the effects on my boiler. There's also some noises from the pipes that (hammer I think) are annoying and quite loud. The central heating is unaffected and works fine.

Also checking the tank in the loft it is still full of water. I haven't detected any leaks so if it is not being used by the flat below it is going somewhere or there's a cistern type valve there to stop it spilling. Either way I'm concerned about this.

Is there something that can be done to resolve this without having to go to the expense of digging up the road and driveway for a separate mains supply?

Any advice would be very welcome.
Thanks in advance.