Wow, what can I say, you are a legend!!
I took guest out of the "nobody = guest pcguest smbguest" option in smbuser like you suggested and then restarted samba. Then it worked straight away!!
Thanks for the "chcon -t samba_share_t /install" advice. As it happened I had already changed that as it has caused me a lot of wasted time google'ing in the past!
Thanks for explaining so clearly. I must confess, I didn't know exactly what the "guest ok = yes" option did. I have taken that out now and applied all your other suggestions.
I also switched back to enforcing mode:
And then restarted the samba service again. Now everything works as it needs to; I can access my samba share as guest with guest via "\\ntinstall\install".