Well I have Avast 5 installed successfully following the advice about uninstalling 4.8 from Control Panel, running the uninstaller tool them running the installation file, rebooting at every step.
Seems OK just now. AvastUI.exe shows 5,928K and AvastSvc.exe shows 60,096K. Does that look healthy

I fancied it was slowing the system up more on a first quick scan, but not sure. OK on the whole and pleased at present.
It's really nice to see a proper scan scheduler

. Great improvement

. I had previously had a daily scan to run Ashquick.exe using the Windows Task Scheduler (which I've now disabled). I'm wondering if I can now set (well I have to see if it works) a quick scan daily for 6 days of the week and a full scan on the seventh day. Maybe I ought to play safe and stick to a full scan as before, effectively.
Windows Security Centre took a little while to realise Avast 5 was installed!
On 'Actions' - what setting are recommended generally if a thread is detected. It seems the default is to do nothing. I've set 'Move to Chest' then no other action for all threats for now.
I really like the professional look to the interface - better than the dinky little media player look of 4.8
Thanks alot for help folks.