It is quite easy to set up the required DNS records to use Google Apps with your own domain for email.
Create the following MX Records:
- ASPMX.L.GOOGLE.COM. 1
- ALT1.ASPMX.L.GOOGLE.COM. 5
- ALT2.ASPMX.L.GOOGLE.COM. 5
- ASPMX2.GOOGLEMAIL.COM. 10
- ASPMX3.GOOGLEMAIL.COM. 10
With 123-reg these follow the same rules as CNAMEs regarding full qualification, and must end with a full stop if the ‘domain’ part is present, (e.g. mail.domain.com.).
You then just need to wait for DNS records to propagate and Google to notice!