Sources close to the London-based firm say it is confident of winning the case over the refurbishment of Bagshot Park in Surrey. Sims is claiming £600 000, although the prince believes he owes only £150 000-200 000. The case is due to be decided in two weeks.
One source said: "We are on the road to proving that we are owed £600 000. The sum should be paid." Sims is also understood to be owed a substantial amount by Southwark council for work on the borough's Canada Estate. Workers employed by brickwork subcontractor Greymount on the Southwark project protested outside Buckingham Palace this week after they were told they would not be paid because the company was owed money by Sims.
However, it is understood that the largest single amount in dispute is over work on Bagshot Park, the home of the prince and his wife, Sophie Rhys-Jones.
A project insider said the original £1.8m contract was for provisional works, but that a number of variations occurred that Sims claims pushed up the cost by £600 000. Although variations are usual in refurbishment work, insiders said the prince refused to accept the amount of work involved in those he ordered.
Mike O'Rorke, Sims' construction director, was scathing in his criticism of the prince in an interview with the Daily Mail at the weekend. He blamed the prince for the project going one-third over budget.
However, a number of project insiders said the firm was naive to present such a large final account.
Sims completed work at Bagshot Park in August and presented its final bill to the prince in October. One project source said: "I wouldn't advise anyone to submit a claim in this way. It stored everything up until the last moment." A Sims insider explained that it was trying to fulfil the partnering ethos by "continuing to work and taking it on trust that they would pay the proper sum when the time came. We were a touch naive. We should have made them pay as we went along." However, sources on the project representing the prince said the Bagshot Park claims were linked to problems on other Sims projects.
One said: "It's true, there were changes of mind by the client, but it was only in connection with the decorating while the wallpaper was being designed and final colours were being decided on.
"The builder did have to jump out of sequence while it awaited a decision from the client on some aspects of the work. But as soon as the information was available, the builder got it. The client wasn't always available on the telephone. He's got a slightly peculiar role in the wider country to do." He said Sims' problems stemmed from a decision to take on too many projects.
"Sims has had a number of changes of management over the last couple of years. Peter Sims quit and that was a shame because he was very good. Andrew Sims, the managing director, had a good team around him but hired Mike O'Rorke, previously with construction manager Schal.
"He [Andrew] brought a new direction to the company, which may have been a mistake. O'Rorke was brought in to bring in bigger jobs – a deliberate move to expand." A spokesperson for project QS Ager and Stockwell said: "We have dealt with CJ Sims on a number of projects and, until recently, we have had a good relationship." The company declined to comment further. Project architect Bowyer Langlands Batchelor also declined to comment.