Ok Will. I'm going to try and explain things about 123-reg and eUKhost and how the things work together. Hopefully that should clear up some of your confusion.
123-reg now only deals with your domain names. All 123-reg does is has the equivalent of a big signpost that say "For domain abc.com, abc.net etc. etc. you need to go to this eUKhost server". You 'wrote' this sign when you changed the nameservers. So primarily, unless you want to move away from eUKhost, you shouldn't really have to change any settings at 123-reg.
Support tickets are basically for getting any help you need from the support team. These can be opened by email at
support@eukhost.com or alternatively via the eukhost
website hosting:
http://support.eukhost.com/.
If you search for plesk user guide or something similar in google, you should be able to get some info about the things in plesk. Or on the plesk
website hosting (not your control panel but the company who makes it - again use google) you can probably see a demo version which you can play about with and not worry about breaking.
On your server, I'm guessing you'll have had an email when you signed up with settings and things for your account. I am personally on Linux hosting so can't comment on the settings but I would guess that your pop server would be mail.domain.com. In plesk there will be some setting for regarding emails and setting up accounts but unfortunatley I can't get on the plesk demo to point you in the right direction.
And I would also thing re: emails that you should be able to get everything from abc.com to forward to abc.net allowing them to use either and you'll get them.
There's no need to transfer the domains as you won't see any benefit.
