Defender won't update

W 8.1 + Avast Free

Windows Update works fine (Notify me and let me choose...) except that some weeks ago, Defender update started failing which didn't bother me as I didn't use it and it was disabled. However I get the message that there are Important Update(s) ready to download every time I switch on, but it's just the one Defender update that fails every time. When I open Defender I get this:



The small window being the result of clicking the START NOW button. Help is no help at all - just says why and how you should use Defender.

I've tried the Windows diagnostics which just seems to delete the installation history and Google gives me lots of results but they involve either a Download (pay to fix which I don't trust)) or just a System Restore, or a tricky looking Registry hack.

It's just a nuisance at the moment as I have to check updates to see if there are any installable ones, but I wouldn't like to try getting W10 with this fault around (or would it fix it?).

I can't pinpoint exactly when it started except it was shortly after the big 3Gb update about 2 months ago.

Any ideas what to do?

Thanks

Defender won't update