Namecheap has just got back intouch with me and it turns out that thier system was holding a different ip address than the one i'd entered as "allowed ip adresses".
For some reason it hadn't updated the allowed ip adresses for my account. Even though i'd entered three different ip adresses and keys trying to sort it out.
For some reason the live api was using an ip address from the sandbox account setup years ago that was replaced at least two years ago to my knowledge.
So for anyone else who has the same issue, make sure you have the same ip address in your sandbox as well as your live environment.
Sorry for wasting your time guys. The Blesta Namecheap module is working fine now.
Regards
Steve
PS.
Bit pissed off with namecheap to be honest, that was a lot of time wasted in checking.