He said: "The restructuring programme has gone slower than expected so we are giving it a renewed kick. We're meeting the management team to decide on the management structure that will take the business forward."
Amec admitted to the City in a trading statement last week that the streamlining of Amec Inc, the Americas business, had not gone as expected, but said they were now gathering pace.
Part of the restructuring involves Amec Inc becoming more selective about the work it takes on. As a result, the business' order book is £300m less than it was at the end of last year.
Amec is reviewing the future of several of Amec Inc's smaller subsidiaries. Siddall said: "We've been looking to see if we should persevere with them but we have not made any decisions."
Mason took over main board responsibility for Amec Inc after Janson's resignation. Siddall said the push to speed up the restructuring programme was not directly linked to Mason's move.
Earlier this year, Amec embarked on a cost-cutting and restructuring drive to prepare for further international expansion and the expected takeover of French firm Spie later this year. The group has split on geographical lines, creating UK, European and American operations. Amec expects these changes to cost up to £5m.
Sidall said the restructuring of Amec's UK arm, which involved streamlining management, had gone according to plan. He said total job losses would be known late next month when Amec announces its interim results for the six months to 30 June.
Siddall said the group was confident that the restructuring would start delivering savings next year, but did not say what these were expected to be.
Amec's statement last week said the group's order book was £4.7bn, about the same level as last year, but made up of more high-value services contracts.