Just switched to plusnet and have lost internet after 4 days.

[#1]

Good point that, will do.
Actually, on Saturday, after it being down for a couple of days I tried an old router that my mam had from when she was on Orange broadband. I didn't have login details for it but hooked it up assuming it would just sync automatically. It didn't work, but could this have caused my problems to get worse?
And the BT test login does work, but as I mentioned a couple of times while on the phone, it doesn't work every time. It sometimes takes me hitting 'Apply' 3 times before it connects with BT test login.

[#2]

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but could this have caused my problems to get worse?

I can't see how to be honest however by default the firmware on those routers is locked down to only allow a connection to orange so it wouldn't prove anything unfortunately.

[#3]

Tried the Thompson 585 router last night and had exactly the same issues.
Router was reset before I tried it, and I couldn't get an internet connection with it. I got a connection using the bt_test@startup_domain username, but again couldn't get one using the bt_test_user@plusdsl.net username. The router was telling me I had authentication problems.
So where do I go from here?

[#4]

Did you wait a good 15 minutes and make a cup of tea/coffee as per instructions para.5(1) just in case that may have worked  
Otherwise this must be a problem with translation between Sheffield and Mumbai 

[#5]

Yeah, left it plugged in and the PC switched on for a good couple of hours, but still no green internet light 

[#6]

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I got a connection using the bt_test@startup_domain username, but again couldn't get one using the bt_test_user@plusdsl.net username

That really does look like the BT RAS is not configured properly and its not accepting the plusdsl.net domain for this line. What confuses me slightly is that Adam said  PlusNet could see authentications for bt_test@startup_domain. I would have thought that authentication for the startup_domain would go to the BT Radius rather than PlusNet, or is it different on 21CN WBC?
Surely though, if it is just a BT RAS configuration error, someone at PlusNet should have been able to check the BTw configuration fairly easily...
So maybe I'm talking rubbish!.

[#7]

I would have said exactly the same!

[#8]

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So where do I go from here?

Sorry to hear there's still an issue with this, from what I can tell this does look like a problem that we may be able to push our suppliers to fix remotely, i.e without the need for an engineer visit. The fault is with a priority team here now so I'll make sure we do what we can to push things in the right direction today. I'd advise keeping an open mind about an engineer visit as sometimes they are needed as a matter of course to allow us to escalate faults further with BT wholesale.
I'll make sure I keep you posted about this today.

[#9]

Ok, thanks for the update Adam.

[#10]

I still think it's BT have mixed up your details with some else's as I suggested here and what PN are seeing is the BT login for someone else on the same exchange. We've seen this two or three times before. You need Plusnet to check if they can see your router in sync/out of sync as you turn it on/off.
Plusnet: Have you investigated this possibility?

[#11]

@jelv I believe we have, the fault has been identified as SSB (server side barring) so it's a case of us leaning on BTW to try and get this sorted without SFI intervention now.

[#12]

Sorted, as per when I called BTW have flexed the circuit for us and I can see an active session now.

[#13]

\o/ woohoo. The missus has just confimred at home that we have a green internet light and can again access the internet.
Thanks to everyone who replied and offered help/suggestions.