Following up my own post...

On the outbound trip there is clearly an OSI and the end result is the
Senior Railcard discounted journey at £3.75 that Barry quoted, though I
didn't see where it was mentioned myself.

I've now found it, rather obviously under the 'railcard' tab of
http://www.tfl.gov.uk/tickets/14414.aspx.

On the way back you were charged the Oyster PAYG off-peak single fare of
£3.90 from Ockendon to Fenchuch Street. Not sure why the £3.75 doesn't
apply in this direction but there you go.

The railcard discounted fare for the appropriate National Rail only ticket
is only £2.60 so that is the figure I should be querying in this case.

It rather looks like an OSI wasn't applied and so your journey from
Liverpool Street to King's Cross was treated as a new single journey. The
fare would have been £1.80 but a Senior Railcard discount on the £14.20
off-peak price-cap gives £9.40 by my calculations and so it was capped to
£1.75.

The TfL site doesn't actually show an off-peak cap for Zones 1-9 + Grays in
the 'Adult' tab of the previously mentioned page, only a peak cap of £18.00.
I had assumed the off-peak cap must also exist and be the same as for Zones
1-9 + Watford which has the same peak cap. I then assumed a 34% discount on
that assumed off-peak cap of £14.20 and that gave £9.40 which is consistent
with the figure quoted for Watford on
http://www.tfl.gov.uk/tickets/faresa...ares/6769.aspx (Grays
not being mentioned) which is quite probably an old page that Google still
indexes on the site. However on the page mentioned previously the figure of
£9.50 quoted by Barry appears for both Watford and Grays. However a £9.40
cap being applied is certainly consistent with the Oyster history shown.

Just to add to the apparent inconsistency, the page
http://www.tfl.gov.uk/tickets/14416.aspx which covers Tube, DLR and London
Overground fares specifies Zones 1-9 + Watford and Zones 1-9 + Grays as both
having peak caps of £18.00 and off-peak caps of £14.50. It isn't clear to me
what a Grays fare is doing on that page at all.

So there are two issues he
1) Should there have been an OSI applied on your return journey?
2) Should the return journey have also been £3.75 like the outbound one?

So that should be rewritten as...

1) Should the railcard discounted fare of £2.60 have been applied for the
journey from Ockendon to Fenchurch St?
2) Should there have been an OSI applied, resulting in a railcard discounted
fare of £3.75 for the whole return journey?

G.