After getting in touch with Pulse8, it would appear the issue is probably at my end (tests say the line is fine).
Could anyone please give me any troubleshooting pointers as I'm running out of ideas?
I have stripped the network down to just the router, which is plugged into the test socket of the main phone socket with a different filter.
The router has been factory reset and the firmware reinstalled. Once the ISP settings have been entered, the router appears to sync (an IP address is allocated), but no service as such is present.
Trying to go online using a browser (have tried via wireless and direct ethernet cable on different devices), it generally brings up a 'page isn't working' message (empty response or timed out), however very occasionally, the browser will connect to a website, and very slowly (slower than dial up) will try to display the page but time out before completing.
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