The solution that worked, that used the module I had already written was

use Package::Alias
    'REST::Client'=>'Finance::IG::REST::Client'
;
[download]

And a rename of my mock REST::Client into my own namespace. I could have called it MockREST::Client or anything really. So now there is no REST::Client in my heirachy, only Finance::IG::REST::Client which is in my namespace, but calling this before the module under test is used, means the module under test uses the mocked version not the real thing.

This means there is no name space permission problem when I upload to CPAN.

Thanks very much for everyones help.