Would you recommend a multiquick flexible extension
No. I would try and avoid them.
Also on the subject of regulations, Section 5(Compartmentation) and section 7-7.6. http://www.planningportal.gov.uk/uploads/br/BR_PDF_ADB1_2006.pdf
You can check with your bco on what is required.
Thanks for the link. The building control guy did mention making it fire proof so I should definately talk to him further, and make sure he's happy with the materials I'm using.