[KevG]

Very doubt that you have a missmash of an engine using various parts, the engines that got supplied to MGR were complete units from Powertrain, they weren't built on the production lines.
It has been known that a few engines from various manufacturers in the past have been slightly over quoted power outputs, a friend had a stock Ford Focus 1.8 Zetec, and that produced 121bhp, instead of the Ford quoted output of 115bhp, and it isn't the first time these things happen, just put it this way you have a very healthy engine to start from, work on the the premise that you have a stock output figure and take it from there, that way you will not be too disappointed when a mod that you have done doesn't live up to what is claimed on the box.
As for your car being built 2005, and having a 54 reg, means that it was probably built between 1st January 2005 and 28th February 2005 (54 reg starts 1st Sept 2004 and ends 28th February 2005)