A System Boiler (SXI) for use with sealed systems where there is a need
to
retain the traditional hot water storage and central heating systems.

The above is quoted from the glow worm website there words not mine

I'm afraid you have misunderstood. This quote simply states that it uses a
sealed primary system. It says nothing about the design of the hot water
cylinder beyond implying that it must be an indirect cylinder. It will work
with gravity fed, pumped gravity, mains pressure (i.e. Megaflo) or heat
banks.

Christian.