Thanks BNM.
Brucey that confuses me even more?!!! I thought Zone U1256 was valid in the Travelcard Zone on National Rail and London Underground services? I know it's not for Buses.
Anyway, I ended up Purchasing a First Advance for Westbury - Stratford London at ^13.85 and Stratford London - Southend Victoria at ^13.00
Total: ^26.85 So slightly cheaper

Thanks for the reminder on the Split Ticket rules. Pretty much every SOV train calls at Prittlewell except on Sundays for some bizarre reason, when I think only a handful do if any!
But on Sundays the SOV services also call at Harold Wood, Gidea Park and Romford. It can sometimes be quicker to change at Shenfield for a London Liverpool Street Fast service!