Yesterday, as his former colleagues counted their good fortune, Mr Clementson was counting what might have been his if he had stayed for a few months longer at the train-leasing company.
All 50 staff, from the board members to the humblest office clerk, are sharing in a pounds 90m lottery-style windfall, following the takeover of the company by the bus and rail group Stagecoach after only six months.
Considering that he has missed out on a fortune, Mr Clementson, who went off to run BR's rail test division in Derby, was in phlegmatic mood. "I couldn't possibly comment," he said, "but how would you feel?". Close colleagues were more forthcoming: "It's a sad, bad, feature of railway privatisation that people can make that sort of money," one said.
"Brian was never against privatisation, but he thought the business was too big to be sold that quickly."
Sandy Anderson, who joined Porterbrook as managing director just before the buy-out, didn't think so, and he was right. Yesterday his shareholding was worth pounds 36.25m while Ray Cork, the finance director, was a more modest pounds 16.74m better off.
Tim Gilbert, who got Mr Clementson's job and his share of the spoils, was looking at a profit of pounds 10, 699,971.12p on his 62,000 shares.
Fate also had a hand in that, as Mr Gilbert, it turns out, was third choice for the post. Mr Clementson was to have been replaced by Andrew Highton, an executive from the engineering group Babcock. But he died in a light-plane crash the day before he was due to start.
As details emerged of the gigantic share bonanza, Labour branded it "the biggest privatisation scandal of them all". Gordon Brown, the shadow Chancellor, said last night that the scale of the profits showed BR had been massively undervalued by the Government and the managers were now cashing in at the taxpayer's expense.
Mr Brown, who claimed that further disclosures were set to emerge in the wake of the break-up of BR, called for an investigation by the Commons Public Accounts Committee and accused John Major of reneging on a pledge to ensure that the abuses which occurred during the gas, water and electricity sell-offs were not repeated during rail privatisation.
"They said they would learn the lessons. Now we know they have learned nothing. Huge profits have been made in only six months as a result of the Government's undervaluing of a train leasing company," he said.
Labour has seized on the bonuses to renew the onslaught against "fat cats". But, for once, the slimmer felines are sharing the cream as well. The pay-out for the 44 staff who were not on the board works out at pounds 390,000 a head.
Yesterday as they slipped out of Porterbrook's offices in Derby for lunchtime sandwiches, the reluctant millionaires proved a shy lot. Not one would talk about their fabulous new wealth, as they had been told not to speak to the press.
The rub is that they have the taxpayer to thank for their unexpected riches. The Government sold Porterbrook for pounds 527m, but Stagecoach has paid pounds 825m. It thinks Porterbrook is worth it because its pounds 1.6bn of revenues are guaranteed for the next 10 years by the government subsidies that the private train operators get to pay Porterbrook's leasing charges.
All is not total desolation for Mr Clementson. He is moving on again to become fleet director of InterCity West Coast, one of the final passenger franchises slated for sale. Stagecoach is bidding for that as well. Who knows, next time it could be him.
Business comment, page 17