Is this a product being manufactured for sale in the UK?
IIRC the Cocoa and Chocolate Products (England) Regulations 2003 don't include a formal definition of "Belgian" chocolate.
Your general obligation will be to not mislead the consumer, in accordance with Article 7(1)(a) of 1169/2011.
Assuming you've validated the claim on the Belgian component (bearing in mind that Belgium sets its own standards for some chocolate products and these are more stringent than the English reg above and European Directive 2000/36/EC from which it is derived, and thus it could potentially be expected to comply with these rather than simply being from Belgium) then in principle I don't see why you couldn't use two different types of chocolate, but you will need to take care with your labelling.
Is there a specific reason you're using the small % of the other chocolate? It would be a lot cleaner in terms of presentation if you could move to solely using the Belgian product, and hopefully the small increase wouldn't significantly affect the overall cost.
You'll obviously need to QUID the Belgian component separately in the ingredients dec either way, but your FOP descriptor based on the current formulation will probably have to qualify that it's "partially made with Belgian chocolate" or similar, rather than the far stronger and cleaner "made with Belgian chocolate".
Either way I'd give your local Trading Standards service a call once you've got an idea of how you want to proceed - most of them now offer assured advice at insanely cheap rates compared to the likes of Leatherhead/Campden, and as it's come from a TSO it can be used if you get any queries/challenges from other trading standard departments around the UK.