I'm travelling from Haymarket to Thorpe Culvert and returning. Because of the very sparse service at Thorpe Culvert, I am going to Wainfleet and walking from there. I bought advances for the Haymarket to Grantham parts, and then an off peak day return from Grantham to Thorpe Culvert, stupidly forgetting that Wainfleet was further on than Thorpe. I asked the ticket office at Haymarket for an excess to Wainfleet, but despite being very helpful, he said he couldn't excess the ticket because there was no difference in price. He told me to speak to the guard. My journey to Grantham was delayed, giving me nearly an hour there, so I tried to get a more concrete form of approval at the Grantham ticket office. (I know the phrase "but the man at the ticket office said I could" holds sod all credibility.) Having explained the situation at the ticket office, the person tried to sell me an additional Thorpe Culvert to Wainfleet single. This would be invalid as the train I was getting to Wainfleet does not stop at Thorpe Culvert. I questioned this, and the person cancelled the transaction and told me to speak to the guard. I don't know what the guard will say as I am still waiting for my train. Surely excessing a zero fare should be possible, even if it just stamping an endorsement on the back of the ticket? Are there any rules that govern the issue of zero-fare excesses (other than "speak to the guard" and "computer says no")? More importantly, if I didn't have a reasonable knowledge of the fare system, I would have been sold an invalid ticket by the person at Grantham. I also point to a phrase used which was "if you get a good guard, he'll let you on". What happens if I get a "bad" guard?