Email merge to custom entity

I have built a custom entity using the 7.3 Advanced Customisation Wizard.

The entity has a selection field and a multi select field.

I have setup the translations for UK and US and triple checked this.

I have created an email template for this custom entity.

When the email runs the items in the selection field and the multi select field show the caption code and NOT the translation.

I am running 7.3 sp1 HF2.

FYI, if I run the same using the mail merge to word, the translations work, so this is entirely to do with email.

Thanks.