Just upgraded to Windows 10 and Microsoft's installer verifies my system and says it's good to upgrade. This should mean all drivers are certified for Windows 10 in theory.
But it turns out the Broadcom Gigabit driver is a major problem in Windows 10, and reading around it seems a number of people have this problem and not just with this model.
Basically if there's a bit of load on the network or sometimes just randomly, the network card just seems to die. Internet connectivity goes to Limited and nothing will fix it other than a reboot. Then it will die again some 5 to 30 minutes later, unless I don't use it and it will stay up.
The driver dates back to 2013 (15.6.1.2) and is the same as supplied for Windows 7. There's no update for the BMC57788 chip beyond that, from anywhere. It seems this is specific to Dell.
I wasn't expecting hardware issues really as the 8300 is a decent and relatively modern spec PC even if it dates from 2010/11. It's been fine running Windows 7, and the spec is way beyond the minimum for Windows 10.
There's a 17.0.0.3 version which is newer from 2014, on Broadcom's site, but while it lists just about every possible BC NIC, it doesn't list 57788, and sure enough it won't install.
Contact Dell support and obviously those "knowledgable" people they hire in India told me the computer is not supported with Windows 10, it is out of warranty and tried to push me to buy a new one, new support, etc. ***!
Lesson learned to never buy this brand ever again, but meanwhile I need to find a resolution as I can't work!