"Graham Murray" wrote

Or even put in a chord, if there is not one already, to allow trains
from the West to access Heathrow directly.

There isn't. There are proposals, as part of Airtrack, for a line westward
from T5, turning south on to the route of the Staines West branch, then
joining the Windsor Riverside line to a new station at Staines High Street
and a new spur towards Ascot. This would allow through trains from T5 to
Reading. Less definite are ideas for a line from T5 to turn north along the
Staines West branch, then to join the GWML by a westward-facing spur west of
West Drayton.

Peter