I seriously wonder what percentage of London to London calls between
fixed lines - which can be dialled without the 020 - actually are
dialled without the 020.

Does leaving out the 020 actually work reliably when you aren't using a BT
line or are redirecting calls via another provider? I always use the 020
myself regardless.

On a sort of related subject, at one London based company I worked for we
were moving to a brand new office and so having a new PABX installed. The
IT Manager hit on the idea of having the code for the outside line be "0"
rather than the usual "9" (or indeed anything but "0") which seemed a neat
trick to me. With a bit of special handling for external numbers not
beginning with "0" such as directory enquiries (which would probably have
been done anyway), it meant you never thought about whether a call was
internal or external, you just dialled it. By forcing all London calls to
be made using the full number it also meant the staff didn't have the option
of getting confused with 020 and 0207/8 and local numbers when the change
came along a few months later. It also gave the supplier's techies
something to think about as they'd never been asked for it before.

G.