Thanks for the pointers - much appreciated.
Just to fill in a few blanks (for anyone else who may be experiencing the same issue) I've got the situation to a just about tolerable state now.
I created a keyboard mapping file using an application called Ukelele to put the double-quotes, "@", "#" etc. into their correct places. That file has gone into ~/Library/Keyboard Layouts/
That just left the Home & End keys. A call to a very helpful chap at Apple Support pointed me to an open source utility called DoubleCommand which allowed me to reconfigure the remaining "special" keys.
Most of the time I access my Mac Mini through VNC from an Ubuntu workstation. The standard built-in British layout seems to be OK in this scenario. I guess the VNC client must be sending the actual characters rather than the key number.
So that seems just about OK, but it does leave me a little irritated for the following reasons:-
1. the Mini is sold without a keyboard or mouse. The assumption would appear to be that you can use a standard generic UK keyboard. This doesn't appear to be the case (if correct operation is defined as having the key symbols mapped correctly). N.B. I've never had this problem with Linux, BSD or even Windows, which all support a wide range of international keyboard layouts.
2. I have a Mac wireless keyboard but I don't use it for anything serious because the keys are in the wrong place (or apparently missing in the case of the "#"). I've been developing software for *nix for decades now - I have no intention of trying to adjust my working habits to accommodate a foreign keyboard layout. "#", "@" & double quotes etc. are pretty frequent key presses in C, Python and Java, XML.
I use a number of different machines throughout the week - it would drive me absolutely crazy if the keys were in different places on one of them. It's a fundamental usability/ergonomics issue, not dissimilar to trying to use a left-handed mouse if you're right-handed!
The OS should be configurable to being used by the whole range of generic input devices - especially something as fundamental as a keyboard. Ignoring the fact that different countries use different layouts is either a serious oversight or a cynical attempt to squeeze a few more quid out of a new customer.
Basically, it appears that a new user cannot set-up a standard working environment on OSX without first getting their hands dirty with creating a key layout XML file. Whilst I'm reasonably happy to do this (if a little irritated) - there's no way I could recommend the Mac to a non-techie user in the UK.
Thanks for the response to the question, as I said, it is appreciated (grumbling aside). A helpful user community is worth a lot, but if I were Apple, I would give some thought to the impact this kind of issue has on the quality of the initial user experience.