Well I've tried the procedure suggested by burakkucat, but no change, the router still wouldn't sync to the ISP.
The router is upstairs on the end of an extension cable so I though I'd take it down to the room that has the BT socket and try it there, plugged directly into the BT ADSL socket. I did this using the RJ11 cable that came with the router as it looked unused. The only difference was that the router tried more to sync, ie. the ADSL light on the front of the router flashed a few times, then stopped, the flashed a few times again and kept doing this. Previously, upstairs, and with the old RJ11 cable, it would just flash a couple of times every now and then. I don't know why this would happen or what it means but I'm taking notice of anything different that occurs in case it triggers anything for me or anyone else. Anyway, still no sync; I gave it a good five minutes of trying.
When I went back upstairs and fired up the old router to get a connection, I thought I would use the new RJ11 cable, but bizarrely, the old router would not sync whilst using the new cable. I swapped back to the old RJ11 cable and got sync as usual. I swapped the cables back and forth a couple of time because I couldn't believe what was happening. Yep, old router with new cable - no sync, old router with old cable - sync. Poltergeist activity?
Anyway, I can't think of anything else to try, (I'm going to test the router on a different line when I can, as suggested by banger) I'm putting the correct VPI/VCI, PPoA and all the other bits in all the right places but it won't sync with the ISP. I have already written to my ISP and asked if there was anything in their logs that would give me a clue, but although they replied and offered to go through the settings with me if I gave them a ring, they didn't offer up any log details.
Here's some entries from the TP-Link router log (see attachment) though, if anyone knows can pinpoint a particular problem from the log entries I would love to know.