@jintymcginty 

I would try this without the homeplugs and see how it works, it could well be the homeplugs are dropping connection and causing issues for you.  Plug the router directly to the extender and see how that works for you, if not drop outs its the homeplugs

After that we can look some more for you, how long does it take to manifest the issue?  is it regular?

@Philbert Thanks for your suggestions, I appreciate your help on this.

I replaced the power line adapter, the one I was using as the Access Point. At that point everything seemed to start working reliably - however I don't take that as proof that it was the power line adapter that was faulty necessarily, as the old one had periods when it worked fine too. 

In the same day, I removed the incoming line attenuator from the line going into the Virgin hub (on advice from Virgin who had apparently analysed the signal) and swapped the  existing Virgin hub for an identical replacement. 

So I don't really know where the problem was, or indeed if I still have an intermittent problem that is yet to recur, but so far so good after 1.5 days with the new components. 

@jintymcginty 

Glad you got it sorted either way, sometimes there is a lot to be said for just leaving it when its working :)