AndreaC wrote:

I hope someone can help me because my little brain is starting to hurt!

I usually use the Picadilly line from Hounslow Central but on the days
when I can't be bothered to walk the extra distance to the tube station
I get on the train to Waterloo instead. I was under the impression that
Oyster isn't valid on South West Trains and because of this mixing of
my journeys I've only used my Oyster for PAYG so far.

However, I am confused by a recorded message that I heard at Hounslow
mainline station on Friday that said something like *- 'please note
that Oyster pay as you go is not valid on any service from this
station' -* thereby implying that an Oyster with a travelcard on it is
valid.

Is that the case?

Yes. An Oyster with a travelcard is valid exactly like a paper
travelcard, that is on all tube, DLR and National Rail trains within the
zones the travelcard is valid.

If so, how would anyone know if you've got a t/c or payg as there are
no barriers or touch in/out pads at Waterloo or Hounslow and lastly, if
an Oyster t/c is valid and I put one on my card, which zone would
Hounslow mainline station be in (given that 2 of the 3 LU stations are
in Z4 and the other is in Z5)

You do not have to touch in/out anywhere when you have a travelcard on
the Oyster and there is no barriers. Ticket inspectors have Oyster
readers so you will not get any problem with them either.

All National Rail stations within Greater London are in a zone (some are
in two zones). The station in your case, Hounslow, is in zone 5. The
next station, Isleworth, is in zone 4.

There is a "connection map" of lines, stations and zones he
http://www.tfl.gov.uk/tfl/pdfdocs/lon_con.pdf

I'm going to lie down in a darkened room now...

If you alternate between tube and National Rail the best thing is
probably a travelcard to the stations on both the tube line and the
mainline you use.

You are not exactly the first person to wonder about these things... :-)

--
Olof Lagerkvist
ICQ: 724451
Web: http://here.is/olof