Thank you John,I don't think you can get any terribly accurate information on build date for a Rover 45 (or a 25). For all the other models, there is the option to enter the VIN at www.mg-rover.info, but the build information for the 25 and 45 is not available (it has been claimed that it is lost, but as the information for both the MG ZS and MG ZR are both available, and those models shared their VIN sequence/serial number range with the 45 and 25 respectively, it seems to me a little odd that the information for the Rover versions would be missing?).
I think XPart have some of the build information, but whether they would respond to an enquiry, I don't know. There used to be a member of the XPart staff who used to post here as XPart02, but he seemed to disappear from the forum at about the same time that the XPart operation was downsized and moved from Desford to Hinckley, so I assume he was either made redundant or was redeployed elsewhere within Neovia.
According to this article on AROnline from October 2008 (News : The hunt is on - AROnline), the highest serial number for a Rover 45 they found was 657793. The very last 45/ZS built was a ZS+ 120 with the serial number 657799, which is only six further on.