Who's invalidated his C of A then, and why? The PIK 'dog doesn't have a C of A - it's very special so has a Permit to Fly.
The Bulldog is a nice machine but of the breed the Pup 100 has better handling qualities (certainly in pitch). There are IMHO better machines out there than the Bulldog. I'd even go so far as to say that I'm now almost of the opinion that it's over-rated.
But then again I've been spoilt by Chipmunks!