Finally sorted. Looks like it was a problem with the Asus router, as it wasn't able to get a DSL link once a connection type of VDSL was selected. I tried a newer TP-Link modem router and it connected straight away with no problems.
Speeds are a bit variable at the moment but hopefully that should settle down over the next few days.
Thanks to everyone on the forum for the help and suggestions. I got much better help on here than through Plusnet's ticket system.
