I came across a very friendly chap when asking to excess a Bath Spa to London ticket to Frome - London. He said that as it was a Zero Excess, they don't normally do it as it flags up in the fares department. Apparently if you get so many Zero Excesses,
FGW▸ think the Guard is on the fiddle.
Source - Guard who I regularly see on W-o-E services!
Could any staff comment on this perhaps?

I must admit that if there is a zero fare excess I normally do not bother. I have to write a written explanation as to why I have done one of these for each occasion. Zero fares are logged and watched very carefully