As I have already said, DLM was obviously reset when you changed ISP. That is why your speed went up. It is pointless getting another reset, even if it was possible, until the reason why DLM kicked back in is resolved.
Should I report a possible fault on my line then and get an engineer to take a look?
Yes but before doing that you should see if there any noise on your phone line? Dial 17070 and select option 2, there should be no noise between the announcements.
Try connecting the VDSLModem/router to the master socket/ test socket to see if your stats improve.
When you use the test socket make sure that any telephone extensions you have are "dead". There should be no dial tone at any of your phone extensions. If they have a dial tone it means you have star wiring and that is not good for Infinity and will need to be fixed.
I have tried the 'quiet test' and there is no noise on the line between announcements. I have the latest BT NTE5C MK4 master socket and faceplate, I don't think there is a 'test socket' on this?
After taking the faceplate off, I will need to plug a filter into the master socket and then the router/modem lead into that alongside the phone socket? Ok, will give that a try, but I will have to disconnect my router and go offline. Hope that doesn't make DLM slow my line down even more!! If that doesn't work, I will report a fault to BT! Thanks for your help.
the DLM does not normally affect the attainabler speed just the conenction speed and as your connection speed is at attainable with noise margin about normal 6db I think it more likely a conenction problem connecting to test socket will be your best conenction so can repost new stats from your tplink please
Here are my latest stats from TP-Link modem router after disconnecting and re-plugging into master/test socket, they look slightly better but still not great!