As a DIY newcomer I have been reading up about plumbing systems with a view
to trying to understand my own plumbing system, but I don't seem to be quite
able to get to the bottom of it....

My 2-storey house has no loft, storage tanks, hot water cylinders or airing
cupboards of any kind. All the hardware is in the garage, which is separated
from the house by a 1 metre-wide path. In the garage there is a Potterton
Netaheat Profile boiler, and a Megaflo unvented HW storage unit. So I have
an unvented mains-pressure system, which explains why all the H and C taps
throughout the house are so powerful - nice!

At the top of the garage rafters there is a small, well-lagged water tank.
The top and bottom of the tank are connected by a pipe loop, and a branch
from this loop feeds the boiler's "flow pipe", which in through another
branch feeds the HW tank (this bit seems clear). Another pipe, this one
well-lagged, from about the middle of the tank seems to disappear under the
ground towards the house.

Things I would like to know:

- I don't quite understand why I need the tank - purely because the boiler
isn't designed to be fed mains-pressure water? All the "unvented systems"
diagrams I have read up on omit tanks of any description.
- Why is the tank so well-lagged? Doesn't all HW go to the Megaflo?
- Clearly the Megaflo feeds the H taps and showers in the house - presumably
it also feeds the radiators? Or might that be the job of the lagged pipe
from the lagged tank in the garage roof that seems to go under the ground
towards the house?

Sorry if this is so basic!

Thanks for any advice.