"he wouldn't be the ONLY person to know about the existence of such a line - hell, accounting should have queried it a LOT earlier!"
Indeed, considering you're talking about a 50k a month spend accounting should really have been on it, but that isn't James' problem.
"Deliberately not telling them when you're AWARE it's going to go unnoticed until it hits the hundreds of thousands is being just as dick-ish as DELIBERATELY making that happen."
Deliberately not telling James that he's up for redundancy as soon as the project's done is also rather dick-ish. If the firm wanted a proper, mature and open debrief then they didn't exactly go about it the right way.
If I were in the same situation I'd keep quiet as well. If, on the other hand the handover was more mature and, quite frankly, pleasant, then I'd not hesitate in mentioning it.