Thanks for your replies...
I've had a go with the 0x prefix, but no joy.
The options when IPv4 are selected are the same as npr's screenshot. The IPv6 ones are removed, but they are further down.
The issue I guess is not knowing what's supposed to go in the IAID and DUID parts. The IAID wants just 8 hex digits and can't be left blank, so I've been using the first eight from the sky user name/password combination and then either omitting or repeating the digits in the DUID box.
I really appreciate everyone's help, but also don't want to waste people's time so I'll probably just go get a router that's proven to work on Sky, rather than trying to get this to work.