Thank you for your replies, guys, and I think it seems that Zeiss gets the thumbs up! However, as I mentioned in the original post, it's too expensive for the purpose I want such a lens for and the next best, price-wise, looks like the Nikkor 105mm micro.
I have John Shaw's Closeups in Nature and he gives very good information about the genre, but as the book was written prior to the advent of digital becoming the norm, much has change since. For instance, I think the Nikkor 105mm micro is now available as af as well as manual, the manual being quite a bit more expensive; does this reflect quality, I wonder, or just marketing? In his book, Shaw prints quite a few shots from the 180mm lens and they are stunning; I do have a 180mm Nikkor but, unlike his manual one, it is af and neither does it focus unaided as closely as I need. There are also restrictions on the type of extension rings one can use with it etc. etc.
Thanks again for your time.
Rob C