Arriving at the ticket booth marked "National Rail Tickets" (operated by First Capital Connect) I asked for an excess on said ticket. The clerk looked confused intially and then called his collaegue over. I was then told that the OPR could not be excessed as it was an off peak ticket and I was travelling in the peak!
Just say "that's fine, I'll travel off peak then, but I want to excess this to Lancaster, can you do that for me please?"
Some ticket office staff do not adhere to the rules of impartial retailing and try to act as RPIs and invent rules that do not exist. The solution is not to get into a debate about which trains you intend to catch!!
I explained that ticket to Lancaster had no restrictions in the evening peak even though it was labelled an Off Peak Ticket. The clerk did not seem to acknowledge this fact, but once again insisted it was not valid.
Well, he's wrong, but he'll realise you are trying to 'beat the system' and some of them then decide to be un-cooperative. Just don't give them the opportunity next time.
He then went onto say that the only ticket valid was an anytime single at a price of £135, thus the excess would be £65 (didn't think it was possible to excess a return portion with a single ticket).
That's right. The entire ticket would need to be excessed to a return if you are changing ticket
type.
Again I explained my position and was told I was wrong as no such excess could be issued in the peak.
You were right. Off Peak tickets can be issued any time and are valid Off Peak. The definition of Off Peak is determined by a validity code. This is basic stuff and the clerk should know it, but I think that he was just making excuses because he knew you were trying to beat the system and did not want to assist you in that!
I did ask for them to check the validity on their computers - they refused. I then asked where they got this information and the reply was that East Midlands Trains had told them that no off peak tickets could be purchased for use in the peak period. True maybe, but only if the ticket has restrictions (the lancaster ticket only has one - travel after 0905 in the morning).
I'd ask who the manager was who said that, as you'll be making a complaint. If no name is given, ask for the clerks name. If that's not given, say you will be noting details of the time, and window that the clerk was using and that you will be reporting the matter that a refusal was made to look up validity restrictions, in contravention of the rules of impartial retailing. Just the threat of it should bring about co-operation

I would also consider asking for a manager there and then.
By this time a large que had formed behind me and rather than keep the heated discussion going I decided to leave it alone and go next door to East Midlands Trains.
I'd have gone to EMT first to be honest. FCC don't expect to do anything other than simple transactions.
The clerk at East Midlands Train took the ticket, looked on his computer, acknowledge the lack of evening restriction and issued the excess correctly which was £1.50 as the difference between the tickets is £3.00. I thanked him for his time and knowledge and asked why FCC could not do the same he replied stating that may have been confused or did not know how to issue correct excesses.
Yep, EMT staff at STP are professional from what I've experienced/heard about.
I spent 10 minutes queing at the FCC booth, a further 10 minutes in discussion with the FCC staff and then a further 5 minutes in the East Midlands Trains ticket hall. Excessing the ticket took 2 minutes!!!
Indeed. If you know how, it's an easy process.
I now know who I should go to at St Pancras for excesses but one thing that annoys me is if FCC did not know how to excess the ticket correctly I would have much rather they say so, rather than try and give me wrong information.
Either they did not know, or were trying to thwart you in your attempt to get round evening peak restrictions. If I was getting such an excess I wouldn't admit I was trying to do that! Not that there is anything wrong with it, but it gives some staff an excuse (in their view - not a valid one) to be un-cooperative.
As an aside when I boarded the train at Euston (it was a Liverpool servicem changing at Crewe) the train manager said no OP tickets are valid on this train.
That's a lie, and he knows it's a lie!
When he checked my ticket he laughed and said "all off peak tickets except yours ... you must know what you are doing!!" It was good to see that Virgin Train Managers are knowledgeable as well.
And Off Peak tickets to many other destinations, or issued with any Railcard! And yes most TMs are knowledgeable; more so in general than booking office staff, especially suburban booking office staff (No offence to the many that are good). The wages of a TM are considerably higher and they are expected to be good at customer service.
Some TOCs do not appear to place customer service high on the list of skills required for their booking office staff, sadly.
This is a little bit of rant I know, but given the booth at London St Pancras states National Rail Tickets I would have thought that this meant all fares including excesses. All I can say you live and learn!
I've had problems with suburban ticket offices too. I remember LM being rubbish at Euston, and I remember FCC at King's Cross being rubbish too. Can't remember the details now. In contrast, East Coast are excellent (although there have been issues reported with King's Cross that have been resolved by speaking to a manager - the London staff aren't generally as good as the others and that goes for on-board staff too).