Peter Masson wrote:

SET don't seem to have any spare stock, and Govia are committed to
effectively the current timetable until December 2009. It is just
possible that as experience with Electrostars builds up their
availability will also increase enough to allow the 508s to be
released, but I would think this unlikely.

Actually it hasn't been that uncommon in recent months to see one of the
three-car class 508 Tonbridge to London Bridge diagrams being covered by a
three-car Electrostar (375/3), so there is certainbly some flexibility in
the fleet at present, although I suspect that it will be as a result of IKF
and the improved performance of the 375s that may eventually allow SET to
rid themselves of the small non-standard fleet of 508s.