Can an excess invalidate a route?

[redengine]

Say I bought a ticket from Bridgend to Melksham (a popular flow, if ever there was one). This would be valid via the sensible change at Swindon route (where there's the added incentive of using Seatfrog to upgrade to First Class), as well as various silly routes, including changing at Bristol Parkway, Bristol Temple Meads, and Chippenham.

Now say I excessed the destination from Melksham to Westbury – still valid via Bristol Parkway, Bristol Temple Meads, and Chippenham – would that mean that the original Bridgend to Melksham ticket ceases to be valid via Swindon?

I'd probably just buy an extra couple of singles, considering the tiny sums involved*, but I'm curious as to what the right answer is in this case (beyond the indubitable fact that the Routeing Guide was drawn up by a numpty).

* SOR BGN-MKM £41.60 + excess to WSB £4.40 = £46.20
vs SOR BGN-MKM £41.60 + two singles MKM-WSB and vice versa £8.20 = £49.80

 

[furlong]

An excess completely converts one ticket into another ticket at the time it happens. Issuing of an over-distance excess is very restricted but a hybrid situation is possible if it happens mid-journey and there is some overlap of permitted routes.

From the TSA:

(a) An Operator may Sell any Excess Fare that gives the Purchaser of it the rights described in paragraph (b) below and those rights are the same as the rights that arise under a Permanent Fare or a Temporary Fare which that Operator is entitled to Sell. However, before it does so it must take reasonable steps to ensure that the Purchaser already holds a valid Fare for the same Flow or (in the case of an Excess Fare which falls within paragraph (b)(v) below) part of that Flow.
(b) An Excess Fare may only be Sold if it entitles the Purchaser of it to:-
...
(v) travel beyond the Station to which the Fare he holds entitles him to travel, but only where he was unable to buy a Fare that entitles him to make such a journey at the Station where and at the time when he started his journey;