When the system is running a small amount of water is coming out of
the overflow from the small black plastic tank in the loft. I was told
the pump may be set too low so I increased it to the second setting
but this only increased the flow of water so I put it back.

Slowing it down should reduce the flow. However, you might not be able to do
so if it is already on the slowest setting, or a slower setting doesn't get
everything hot enough. It probably happened when you changed the pump as it
was marginal before, but the new pump is just a little more powerful, enough
to pump over.

Quite frankly, systems with loft tanks are a complete pain in the arse. You
could fiddle around for years with air separators, moving pipework,
adjusting valves and pumps etc. Or you could rip it out and replace with a
proper sealed pressurised system that simply doesn't have all these
problems. There are other advantages too. It is much easier to bleed
radiators and flush the system. You get more storage space in the loft.
Leaks are less likely, as you can remove pipes from the freezing loft space.
Any leaks you do get are much less serious, as there is a limit to the
amount of water lost due to the manual fill arrangements.

To make the change, you just need a kit to upgrade, which will include a
pressure vessel, pressure relief valve, filling loop and pressure gauge. You
install these items, rip out the tank and you're done. It is a relatively
simple DIY plumbing job, similar in scope to plumbing in some new taps.

You would need to ensure that your existing system is compatible with sealed
operation. This basically means your boiler must have a safety cutout.

See:

http://www.plumbworld.co.uk/acb/show...=3479&CATID=17

(in case of wraps
http://makeashorterlink.com/?D51A35C67

The exact size you need depends on the size of your system. They have kits
from 8 litres to 24 litres. If you know the number and average size of the
radiators, you can calculate the minimum size required. Using one that is
too big isn't a problem. They vary from 40 to 50 quid including VAT
depending on size.

Christian.