The Conditions of Carriage have this to say:
"(e) If you make your journey by a route other than those referred to in (a) and (b)
above, you will be liable to pay an excess fare. This excess fare will be the
difference between the price paid for the ticket you hold and the price of the
lowest priced ticket(s) available for immediate travel that would have entitled you
to travel by that route."
From Condition 13.
"Penalty Fares are charged by Train Companies at some stations and in some trains. You
may be liable to pay a Penalty Fare if:
(a) you travel in a train without a ticket or Permit to Travel; or
(b) you travel in a class of accommodation for which the ticket is not valid; or
(c) you travel in a train and the circumstances set out in any of Conditions 10,
11, 12, 18, 19, 22, 30, 35 and 39 apply; or
(d) you are present in a Compulsory Ticket Area without a ticket or Permit to
Travel"
http://www.nationalrail.co.uk/system/galleries/download/misc/NRCOC.pdf
I can't see how a penalty fare can be charged for being off route. I suspect it was treated as travelling over distance (Condition 18) but I think the correct excess would be thie difference between the totasl of a Hatfield-Finsbury Park and Finshury Park-New Southgate ticket less the fare already paid for the Hatfield-New Southgate ticket.
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I would also add the condition that doubling back between Hatfield and Finsbury Park is allowed only if the train between Finsbury Park and New Southgate does not call at Hatfield. The routeing guide does not contain this restriction, but I base my opinion on the contract for carriage being completed on the first arrival at the destination.
I think you mean the Hatfield-Finsbury Park train not calling at New Southgate, as Hatfield is further out than New Southgate. Also, I believe you are treated as ending your journey when you leave station premises, so I'm not sure whether that applies.
What I would say is that the National Routeing Guide in Detail section seams to allow lots of bizare things (and dissallows sensible things) as it does not seem to be writted with much thought for the rest of the guide. I tend to stick to the main guide (the bits John has highlighted in red).