Just a quick update - the VR2800 was stable for another few days so I bit the bullet and put the Sky router back in and did some automated online tests, it picked up router was below min speed and then said it was going to do some further testing. It then popped up and said you need an engineer visit and it then booked me in for the 27th October.
I found out relatively recently on my NowTV (essentially Sky) ADSL connection that if they notice something amiss (in my case it was many CRCs and frequent disconnections), and you are using the supplied router, they allow you to run Openreach's copper line TAM tests from the "My Account" pages. When I did this, the test failed with a 'CIDT fault' (though didn't actually tell me this) and they asked me to ring them. It may be that the copper line test has detected something more conclusive in your case, hence they have gone straight to an engineer appointment without you needing to go through the usual troubleshooting steps (test socket, different filter etc) over the phone.
So Sky router left in on the banded 35/20 profile, I am hoping the engineer might be able to see why the line has been banded but it is a bit of a wait.
Yep, if the TAM copper line test is finding a hard fault I'm sure the visiting engineer will fix it and then reset the DLM for you.
the VR2800 is not compatible with MER/Option 61 when in wireless router mode only so I have decided to return it and look at other options. The VMG8924-B10A is on it's way, and interestingly I think I could use it as a modem with my SR203 as the Sky router has WANoE mode.
How annoying, a good decision then to send it back. Indeed, the Sky routers do support WANoE as they originally had to work with the Openreach supplied VDSL modems, and now need to work on FTTP. Not sure why you'd want to do this though, the VMG8924-B10A is likely better in every single way as a router (assuming it works with the patched firmware I linked to earlier, which I'm pretty sure it will).