National Rail Enquiries said:Changing the time or date of travel
Refunds
Changes to time or date of travel must be arranged before departure of the first reserved train printed on the ticket, after which the ticket has no value and a new one must be purchased. You will need to present the ticket(s) and reservation(s) when you request a change. Changes to tickets cannot be made on-board the train. If you board a train without a ticket and reservation for that service, a new ticket must be purchased. Depending on the Train Company you are travelling with, you may be liable to a Penalty Fare if you board the train with an invalid ticket.
The origin, destination and Train Company or route shown on the ticket(s) must remain the same. The difference between the price paid and cost of the next suitable fare for your journey is payable, plus a £10 administration fee per person, per single ticket for each change to a journey. If you change to a train on which a cheaper fare is available, the difference will not be refunded.
Your ticket is non-refundable.
Your best bet is a Walk up off peak day return Laindon-Billericay
... break of journey is not permitted on Advance tickets. I am not necessarily saying that it is either enforceable or enforced in every situation but we can't be seen to be recommending breaching the T&Cs of the ticket. If the T&Cs are unclear, then that's a different matter, but in this case they are clear.I think I know the answer to this but strictly speaking....