Thanks for your reply COLgeek,
I left the PC on overnight disconnected from the web, ran DISM then sfc, upon checking just now I found that Avast has successfully installed, how long it took I have no idea but many thanks for your response.
A bit of background is that I had Bitdefender free installed for some time with no problem, then it was updated to the online free version which failed to install repeatedly. I uninstalled Bitdefender but could not get Windows Defender to run properly it only would run periodic scans hence the need for another AV program.
Cheers