I think the test on this situation is whether or not a first class fare is available for use on the line.
Now a check of NRE elicits the fact that there is NO first class fare for the route Norwich to Sheringham, HOWEVER, this needs to be qualified by the fact that NXEA DO operate a Weekend First supplement for travelling first class. The terms stated indicate this is only available on Saturdays, Sundays and Bank Holidays. Because it is quoted as a "supplement" rather than a fare as such, this would give it a wider applicability to cover journeys where rolling stock with First Class accommodation is provided, even though there may be no fare.
I know saying this will be unpopular and will no doubt generate a lot of criticism however the facts are that there IS a supplement payable, and it DID apply on the day that the OP travelled. The Guard was quite correct in that she had the right (and indeed should have to act correctly) charged the First Class supplement.
As there is no supplement payable Monday to Friday, then there is no excess charge that can be raised as thus there is no specific issue in sitting in First Class. The test is CAN an excess charge be raised - in the case of Mon to Fri there is not and thus there is no fare avoided.
DaveNewcastle is correct in his determination of the situation with First Class. I would just add in support of his post the arrangements that applied under BR, and still do which is that technically unless First Class accommodation is declassified, the fact that the train may be running on a service for which there is no First Class accommodation specified in the timetable does not ordinarily confer the right for a passenger to occupy a First Class seat with a Standard Class ticket.
Where rolling stock is filling in for a normally Standard Class only service is running with First Class vehicles, then the stock should be declassified or a suitable announcement made permitting the use of the First Class facilities where the loadings suggest such a course of action is desireable.
Wherer rolling stock is operating on a line where there is no First Class FARE and thus service, then obviously there is no need to declassify, as there is no First Class fare and no excess charge can be raised (except as pointed out earlier).
Note, the indications of the seating classes shown in the timetable are for indicative purposes only, and are intended to indicate WHICH services are booked to run with First Class accommodation so that passengers wishing to travel First Class may avail themselves of this facility. As mentioned above, this does NOT give a carte blanche to travel First Class on a train which is shown as being Standard Class service only on routes where First Class is provided.