Katamarino,
I have experience with this, and perhaps can be of help. I do these types of approvals on Canadian registered aircraft, but might have some insights which would be of use to you with the CAA. I also know of two unused 65 gallon tanks which are looking for a new home. I recently made the restraint system for them, but now it's too late in the season to use them. PM me if you wish...
Pilot DAR