After a lot of searching, I have now found an image of the Whipps Cross memorial.
This includes a dedication to the 59th (The Essex Regt) HAA Regiment RA.
Originally located in Walthamstow, this monument was moved in the 1950s to Whipps Cross, just outside the Territorial Army Barracks, Lea Bridge Road, Greater London (E17 3PA).
This must be in/near the site of the Cadets Training Centre, where dad and his pals met up each year for their annual reunion.
Note: these images have been re-worked from original photos by David Anstiss
At the bottom, the engraving includes the following message:-
WE ARE THE DEAD.
TO YOU WITH FAILING HANDS
WE THROW THE TORCH;
BE YOURS TO HOLD IT HIGH.