You can't patent a design.</pedant> (You can *register* a design in the UK, but I don't believe that has international applicability. You can probably indirectly patent designs, if you've got a sufficient wad of cash to blow on patent attorneys.)

Surely a design is capable of also being an 'invention' - eg, device for dispensing instant coffee with minimal spillage, or similar?

Yes, a "method of minimising wastage during dispensing of a granulated substance" or somesuch probably would cover the design of any crucial subcomponents used in the process. It just has to be expressed in the right way; you're not patenting the design per se, you're patenting the invention. (It's probably more subtle and can be better expressed than I've put it.)