"Martyn Dawe" wrote
I believe these trains are based at Ilford does this mean that at
the
start of service they have to come into Liverpool street and out to
cross the lines to get to the chingford side. where does this
happen ?

At Liverpool Street, platforms 1 to 4 are accessible from/to the
suburban lines only, platforms 5 to 10 suburbans or mains, 11 & 12
mains only, 13 & 14 mains or 'electrics', 15 to 18 'electric' lines
only. There are various places trains can cross e.g. around Bethnal
Green, or between the electric and the main lines further out.