Thanks for the reply guys,
Symphony, I wholeheartedly agree with you about the locker needing to be gas tight against the rest of the boat, just really surprised that no one (or company) has made a slightly bigger locker that can accommodate a 4.5kg bottle and still uses the original space as imha there is a lot of wasted space between the under side of the cockpit seat and the top of the original gas locker!
Sy anniina, are the bottles in a gas box or just strapped to the bulkhead?