do Avast use Digital River to manage payments on behalf of Avast Tech Support?
http://www.avast.com/en-gb/total-support.................. is this or is this not genuine Avast?
Yes to both.
I'm a bit confused.
Is it the case then, that Avast has dropped the 3rd. Party Support Technicians (known as guruaid) they were contracting and is now providing it's own support technicians. 
I see you're really confused, as Avast have never had a contract with GuruAid. Where did you come with that idea from?

We are aware that GuruAid announces themselves as "official Avast support", but that is not true, just as many other fake support pages that advise them as that.
Avast Total Support is the only official support provided by Avast. Even though the call center is not located in Avast HQ, they are indeed as
DavidR said "a 3rd party support outsourced by avast. Whilst it may not be avast, it is commissioned by avast and they have to take responsibility for it". The same way I could say that when I called to Amazon the last week I didn't really talked to Amazon because their call center is in India and not in-house.
So
jonnywill1661 and
cridgejm, if you contacted Avast Total Support to the numbers indicated on the webpage (
http://www.avast.com/en-us/total-support or
http://www.avast.com/en-gb/total-support or for another country), you have indeed contacted the only Avast official phone support, and they are contracted by us to present themselves that way and to provide that service. For any issues with the call center please attempt first to contact them by phone directly or submit a ticket to us
https://support.avast.com/Tickets/Submit