Unfortunately they won't accept either of those products...
On the multiboard:
"I've checked the British Gypsum website and the only semi exposed applications the board is acceptable for are eaves, canopies and carport underlining. I presume, therefore, it is not recommended where there is a risk of moisture penetration through any cladding.
Please ensure that any board achieves the 30 minutes period of fire resistance and is adequate for the proposed application."
On the Hardi board:
"This backer board is more suited to internal applications in wet areas. The BBA Certificate only meets requirement B2. It needs to meet B3 to demonstrate the required level of fire resistance."
I am asking British Gypsum direct if the Multi-board can be used under the FC cladding.