Thanks ALL of you for your helpful links, info on Shires & Essex and
timetables and the like. THANK YOU.

I would suggest taking the Tube from Stratford to Epping (30 minutes)
and then the 501 towards Brentwood (about 20 minutes).

I can get a One train from Stratford to Brentwood and get the 501 from
there too. I guess that's faster? or maybe not.