I would advise against those Advance fares.
Get a walk-on fare instead, then if you decide not to travel there is no cost (unless you bought a walk-on fare in advance, then there is a £10 admin fee). There is also no cost if you change travel plans.
'WALK ON' PRICES:
If you just want to go to London, a Super Off Peak Return (SSR) is priced at £
40.75.
This must be bought before boarding the train if you catch East Coast, but can be bought on the day of travel. You can buy on board Hull Trains
Outward & Return valid on ANY train on Saturdays.
However if Hull Trains timings suit you, then you can save a few quid by getting a Hull Trains only ticket for £
33.65 This can be bought on the train.
Outward & return valid on any HULL TRAINS serviceon Saturdays.
If you wish to travel round London then get
Retford-Grantham SDR £11.10
Grantham - London Zones 1-6 Travelcard £23.50
TOTAL £
34.60
The train must call at Grantham for this to be valid.
Most trains that call at Retford also call at Grantham. Check times before choosing this option.
You can buy
both these tickets at Retford before you get on the train. If you use Hull Trains you may buy on board.
Retford station is not obliged to offer this combination but if you ask for it they must sell it.
TIMES:
East Coast (Sat 27 Nov)
http://www.eastcoast.co.uk/Documents/sat271110ptt%20offer.pdf
Hull Trains (normal saturday service)
http://hulltrains.co.uk/timetable.php#satsouth
From Grantham: 0820 (EC),
0910 (EC), 0921 (HT)
From London: 1840 (EC), 1937 (HT), 2000 (EC)
Note: 0910 does NOT call at Grantham
at 27 Nov 2010 Changes:0 Duration:1:40hrs