Thank you, Elwyn, Glensman and kingkerswell for your quick replies. I'm sorry I'm late getting back to you, but have had a family crisis and have only just looked at my messages.
Yes, Isabella's fathers name was definately Samuel Harpur, but if what you say Elwyn about those distroyed records, then it seem likely that I'm not going to be able to find the right birth.
Many thanks to you all, and I'm sure I will keep "dipping into" the research sites from time to time, just in case!
Chris.