George Reid, the Presiding Officer, announced yesterday that the fiasco over the broken beam was caused by a 1.50 steel bolt and a workman earning 25 an hour who twisted the bolt so far the wrong way, the head almost came off.
The beam, which swung loose at one end two weeks ago, should have had two locator bolts at the bottom and two at the top. But a report from the structural engineers, Ove Arup, revealed that there was only one bolt at the bottom and this had been jammed into the hole in the wood.
Obviously aware that the bolt was jammed, the workman had tried to get it out, either by wrenching it anti-clockwise, or by bashing it with something, and this almost took the bolt head off.
The other was missing, has never been found, and the engineers now presume it was never inserted in the first place. So when the head on the one inserted bolt finally broke off, the beam swung free, held on only by the bolts at the top end.
But Ove Arup found even more problems at the top end. They discovered one of the bolts at the top end was the wrong size and was too small to fix the beam in place.
Aware that there could be similar problems all over the roof, Ove Arup have now decided to place steel collars round the points where the timber beams come together. This will act as a "belt and braces" approach, supporting the beams and keeping them in place just in case any more bolts break.
The engineers will work on a temporary solution over the next few weeks, which they hope will allow MSPs to return to their debating chamber by mid-May, at the latest, and they will then work on a permanent solution through the summer, completing all the repair work on the debating chamber roof by the time parliament returns from the summer recess in September.
Mr Reid yesterday described the Ove Arup report as "real progress". He said: "We now know the likely cause and have discussed with Arup their favoured option of how to resolve it - additional components around the bolts."
Mr Reid said the only other option would be for the engineers to remove all the bolts in the roof, to find out whether any more had been put in badly, but this would be a huge job, taking a long time, costing a lot of money, and it might not achieve anything.
Philip Dilley, the chair of Arup Europe, said the bolt and the beam had been examined by a laboratory in St Albans and the scientists there had "confirmed what we had suspected for some time".
"The bolt head was twisted off. The bolt was jammed into position and was removed by someone trying to take it out again," he said.
Mr Dilley said the bolts had been ordered specifically for the parliament and he did not know how much they cost, but similar M10 socket head, cap screw bolts can be bought online for 1.50 each.
Evidence that the debating chamber fiasco has been caused by a construction problem will help the parliamentary authorities in trying to apportion blame and to get the money back for the taxpayer.
The two-week move to the Hub, at the top of the Royal Mile, cost 120,000 and the parliament is likely to be in committee room 2 for at least four weeks, and possibly six, which will cost between 64,000 and 80,000. The total cost of the move will come to about 200,000.