I'm Nerys and I work here at 123-reg.
I can see from your post that you wish to keep your email address with 123-reg and host your website with Moonfruit, is that correct?
If so, to do this you will need to keep the domain name and the nameservers with 123-reg. The MX records will be the default 123-reg MX records (which, unless you have changed since purchased, should be set by default on your domain name). This will all keep your email working correctly.
To then have your domain name pointing to your website created with Moonfruit, you will need to point the A records for your domain name to the IP address of your hosting package, which Moonfruit will be able to supply you with. Once you have the IP address, please use the guide below to set the A records for your domain name:
If I can be of any further assistance please do ask.