Thanks for the replies; I agree with what you say. There is no way such a simple-minded tool could cope with many different 'house' styles around, especially the older ones. And I'm only using it on those ebooks where a half-baked conversion has been done, leaving a nasty mix of straight and curly quotes. These are usually conversions to etext from books published from around 1970 onwards. (often those with the title and author centered between rows of hyphens on page 1).
The single quote section was a bit of an afterthought, and I'm still dithering about whether I will retain it.
I used 'Cristo' as the example as this happened to be the largest book I had conveniently available, and gave a good idea of the runtime. The usual book normally runs through in less than 5 secs.
I clearly misnamed the thread. 'Checker' or 'correction' would have have better conveyed the intention rather than 'converting'.
Still, there it is. I leave it to the reader whether to use it as another tool in the armoury, to use it as a framework to build something better, or to use it as an awful example of how not to do it.