1930's RA TA South London

Good Afternoon Everyone,
I'm currently F,F,@ FFH! I'm trying to trace what RA TA unit me old man served with from 1936!
I know he lived in Eltham. He worked in the danger buildings in the ARSENAL [ motto :- If in doubt DONT ! ]. I know he was in France, and got out of Dunkirk on the Thames dredger "Gallions Reach " I know he went back, with 86 Field Reg. Hertfordshire Yeumanry.
That's it . I don't know when he joined the H Y, or who he was with in the earlier days
Any help or advice would be very much appreciated.