You're right, that the new Site Domain config page (in conjunction with 123-reg or other registrar) allows you to do everything you need.
The first thing to do is to add www.abingdonhydro.org.uk as a domain using the Site Domain page of your site.
The site will record that you want to use this domain, but it won't actually use it yet until you set up the DNS on 123-reg.
To do that, in the 123-reg control panel for your domain name, go to Manage DNS. Then you want to add an A record for www to point to the IP address of the Voice server - 18.104.22.168. (I'm not sure of the precise links you have to click here, but that's the gist of what you need to do!)
It will take a while for this change to reach everyone, but within a few hours (sometimes up to 24) it will be in use for your site.
We don't do anything with email - you can configure this independently of Voice using the 123-reg control panel.
Thanks, I did it (more than 24 hours ago) but I must have done something wrong because the web site has disappeared! Both the old address e-voice.org.uk/abingdonhydro and the new address abingdonhydro.org.uk go to the e-voice home page. I did get into my site, twice, by logging in to e-voice then going to it through Your_Account/Groups, but usually that did not work either. The two successes were using Linux/Firefox - no success with Linux/Opera. I checked the Site Domain page and it reported that the abindonhydro.org.uk redirection was OK. Is there something in my web site that e-voice doesn't like...?
There was a subtle problem, the fix is that I've gone to your site config and changed the domain name setting from "abingdonhydro.org.uk" to "www.abingdonhydro.org.uk". i.e., the same but with the www.
The version without the www is having a redirect done on it by 123-reg, which is what was causing the problem (because Voice was redirecting to the non-www version, then 123-reg was redirecting back again in a loop!)
Telling Voice to use the version with the www means it all works fine.
A PS to the question above: the fix worked, for which thank you, would never have found it. However it led to a further mystery, that it worked with all browsers except Firefox. The solution turned out to be simple - Firefox was still looking at the old address, so it was solved by emptying the cache,