As far as cold is concerned, turn off the water at the main stopcock
(probably under the kitchen sink) and open a downstairs *and* an upstairs
tap. The pipework will drain through the downstairs tap, with air being
admitted through the upstairs tap.

It's an upstairs flat!

With the hot system, you have to remove the pressure by stopping cold
water
getting from the header tank into the hot cylinder. Hopefully there will
be
a tap or gatevalve in the cold feed to the cylinder (the one which comes
down from above and goes into the *bottom* of the cylinder. If so, turn
that
off and then open an up and a down hot tap - as for cold - to drain the
pipework. If there *isn't* a valve in the cold feed, you'll need to reach
down into the header tank and stuff a tapered cork into the outlet pipe.
[Choose one of a size which leaves enough to catch hold of to pull it out
again when you've finished!]

It's a combi boiler (sorry, didn't tell you that bit)

Cheers though! ;-)

Andy