Thanks again for all the replies.
It does now work ... sort of. I can browse the shared areas using the IP addresses of the home PCs. Still can't get the names of the PCs to work properly, but that's irrelevant - I can map a network drive using the IP address and job is done (I have fixed the IP addresses on my home PCs and Mac that I want to share from).
Trouble is currently that, in order to see the shared areas on my home (XP) PC, I seem to have to set them to be wide-open for anyone. Full permissions to All under the "Sharing" tab, and also full read/write access to "Everyone" under the Security tab. I can't find a way under the XP PC to set the sharing & security so that only my Win 7 laptop, for example, can have access to the shared files. I can restrict it to users on that XP machine, but can't seem to do it externally ?
I never used to worry about this. With my XP Domain Laptop (Work) I used to just map a drive to my "Home PC\c$" and then navigate from there. Full read/write access was granted even though I hadn't shared the drive - all I had to do was authenticate with my username & password for my home PC. I was a bit happier about this as I didn't have to "share" my drive, or allow control to "everyone".
I'm probably missing something obvious here. Perhaps I'm getting too worried over security - after all, if I could get in before without allowing it properly in the "Security & Sharing" settings, then so could anyone ?
Thanks for all the help so far. Any further thoughts on this would be appreciated - in one short spell on here I have resolved an issue that has been bugging me for weeks.