This is called a "System" boiler (short for "Sealed System).  There's an expansion vessel and no cold water storage tank.

It should run the hot and cold taps at about 3 bar (pressure reducing valve to control this).

Inhibitor can be put in through a rad bleed with a special gadget (rad turned off at both valves and drained first to allow space).

System boilers are the way to go imho.

An expert will be along shortly to say I'm completely wrong  

Sounds similar to what we have except we have the water tank is underneath.  

Know as a mains pressues system.

The gauge probably measures the pressure in the rad circuit and is usually around 1.2-1.5 bar. There is a valve that when opened allows mains water into that loop to adjust the pressure (we never have to touch it).

For hot water... Mains pressure water comes into the house and direct to the tank under/over the boiler. No header tank required as the whole hot water system operates at mains pressure. Somewhere there should be a relief valve so that if anything goes wrong (say the mains pressure increases or the boiler boils the water) the whole lot won't explode. The relief valve is usually set at around 3 bar and should have a discharge outlet somewhere.

My guess is that the old tank is just that - an old one not in use since the boiler was changed.

I believe "system" essentially means the pump for the primary and rad loop is built into the boiler so you don't need one elsewhere.

Colin (not a plumber)