Embarrassing!!!
I nearly got this right. I just needed to restart Visual Studio when I changed the project type Guid
So - the project type Guid for MVC4 is
{E3E379DF-F4C6-4180-9B81-6769533ABE47};
and the full element is
<ProjectTypeGuids>{E3E379DF-F4C6-4180-9B81-6769533ABE47};
{349c5851-65df-11da-9384-00065b846f21};{fae04ec0-301f-11d3-bf4b-00c04f79efbc}
</ProjectTypeGuids>
BUT - don't forget to restart Visual Studio (Doh!!!)
I'll keep this question up in case others have got the same problem
Thanks for everyone's time