Thanks for getting back to me, sorry I've not answered sooner - I
turned on topic watching but for some reason I didn't get notification
of new messages.

When we bought the house a couple of years ago the previous owners
asked us if we would like a lesson on how the heating system worked but
we declined as we didn't realize it was anything special. I wish now
that they had told us what was involved. They did at least leave some
notes tied to various pipes which gave us an idea of what to do.

I know enough to keep the system topped up with water (but thanks for
the reminder because it needs doing - how often should it be checked?).
A note tied to it suggests it should be filled to 1 bar so I fill it to
that (+/- 0.1) when the system is cold. When it is fully hot it goes up
to maybe 1.75 at most and there seems to be an emergency pressure valve
set to 3 bar on top of the red cylinder. I've posted some photos of the
system here

http://www.crazysquirrel.com/images/heating/1.jpg
http://www.crazysquirrel.com/images/heating/2.jpg
http://www.crazysquirrel.com/images/heating/3.jpg
http://www.crazysquirrel.com/images/heating/4.jpg

As you can see the honeywell timer box has been bodged to the wall
using a standard electrical box. I think it must be held on by a piece
of twisted wire or a zip tie - I have been unable to get it off the
wall dispite the fact it wobbles everytime you touch it so I don't
think it is a screw holding it on. The wobblyness is why I suspect
there may be a loose connection on the back of it.

In answer to some questions raised above:

The large (1inch?) pipes running to the boiler come in the top and can
get visciously hot when the heating is on so I don't think it is a lack
of water.

There is certainly no header tank - the system is sealed (I believe
unvented is the technical term)

I spoke to a plumber (who never showed up - grrr) on the phone when the
problem first appeared. He mentioned the bubble at the top of the hot
water cylinder and I have read what it says on the side of the cylinder
about establishing the bubble. It sounded sufficiently difficult that I
decided to leave it alone (I will try most DIY jobs but I know when to
leave it to a pro). There isn't a problem with getting hot water out of
the tank so I think the bubble is ok if I understand the system
correctly. The tank does need cleaning though as we sometimes get a
little dirty / muddy water out when we first draw water after heating
but I am happy to leave that for now. I would like more information
about this system - I had a look on-line but coudn't really find much.
I have done work on vented systems but this is a totally different ball
park.

I would be really supprised if low water pressure was totally to blame.
Even when the the control box has both switches (bottom right in photo
1) flicked to off and the lights are off the boiler sometimes comes on.
I am fairly convinced that the cavatating of the pump (and hence the
noise) is then caused because the three way valve is set in the off
position so it is pumping against the valve. When this is happening
(and thankfully it's been fine for the last few days) we don't get any
hot water or heating but the boiler is going 10 to the dozen.

Many thanks