This shot is of Southend PD3 313, PHJ952, one of four Southend PD3s converted to open top.
The four had their offside sunken gangways covered over on the top deck, to give a highbridge layout (see top deck shots of 311 and 313) but downstairs retained a pure lowbridge layout to the end.

You can see on the seats on the right, the 'Please lower your head when leaving your seat' warning sign found on all of Southend's low bridge double-deckers.

Photo taken by Richard Delahoy, Sunday 27 August 1978