Booking for August will not be available until approximately 2 to 3 months prior to departure date.

If you look on atoctravelagents.org and click on Booking Horizons it will tell you when bookings are currently open to (give or take a few days), do not bookmark Booking Horizons page as that URL changes

I think all the summer reservations will be released at the same time.
They are currently available to July 9th.
And book as soon as they  come out, don't delay even by a day.

According to a few people I know, reservations will probably release in a few weeks for half way through August.

As intrepid Agatha christie fans, we are going to look at the sleeper London to Penzance. Any comments?

The advance fares for First Great Western priced rte AP SLOUGH (route 411) on London to Newquay are as follows:

Codes in brackets are the ticket sale code and someone can probably correct me on the lettering!

All restriction codes state "Valid on date, FGW train and seats shown on the ticket, plus connecting trains as required." - obviously there are no connections so that last bit does not apply.

(DES) 1st Leisure Advance E (1st Advance D I guess...) - £44.00
(DCS) 1st Leisure Advance C (1st Advance C) - £77.50
(DBS) 1st Leisure Advance B (1st Advance B) - £94.00
(DAS) 1st Leisure Advance A (1st Advance A) - £110.50

(DHS) 1st Business Advance C - £100.00
(DGS) 1st Business Advance B - £126.00
(DFS) 1st Business Advance A - £153.50

(WES) Leisure Advance E (Std Advance D I guess...) - £15.00
(WCS) Leisure Advance C - £26.50
(WBS) Leisure Advance B - £32.50
(WAS) Leisure Advance A - £38.00

(WHS) Business Advance C - £71.50
(WGS) Business Advance B - £90.00
(WFS) Business Advance A - £110.00

For what it's worth, open fares are as follows:

NEWQUAY 3523
rte Any Permitted 
GWR      (S)     (R)
FO     179.50  359.00
ET             370.00
FS WX          218.00
FO     128.00  256.00
SV WX   72.00   73.00

Restriction code WX is as follows:
WX
JOURNEYS TO/FROM/VIA LONDON/READING
Available as listed below MONDAYS-FRIDAYS (By any train on other days)

TOWARDS LONDON - Available on the following trains and ALL later services:
Penzance 0542, Camborne 0602, Redruth 0609, Truro 0621, St Austell 0639, Par 0646, Lostwithiel 0652, Bodmin Parkway 0659, Liskeard 0712, Menheniot 0718.

FROM LONDON - Only valid on services departing London Paddington or London Waterloo between 0903 and 1635, or after 1800 (de-
parting Reading, heading west (that is services passing through Didcot Parkway, Oxford or Newbury ONLY) between
0928 and 1706, or after 1830).
On Fridays, also valid on 1703 departure from London Paddington (1732 from Reading.)  Also Monday to Friday not valid until the following trains from the following stations, connections into the following trains ARE valid.
Bath Spa 0913
Bristol Parkway 0932
Bristol Temple Meads 0930
Cardiff Central 0855
Castle Cary 0908
Cheltenham Spa 0831
Chippenham 0925
Exeter St Davids 0823
Gloucester 0846
Kemble 0919
Newport 0909
Newton Abbot 0803
Pewsey 0811
Plymouth 0725
Stonehouse 0859
Stroud 0904
Swindon 0935
Taunton 0851
Tiverton Parkway 0837
Totnes 0750
Westbury 0817

I think I've understood the restriction correctly!!! 

The 1105 Padd - Newquay is usually worse than the earlier one in terms of the clientell that use it, i.e. the 0905 is too early for all the teenagers going on the pi*s!

Thank you for bringing that error to my attention - my mistake when I set the board up, and I take full responsibility.  Sorry - spelling now corrected.