Fixing Plasterboard, Battens and Insulation to brick

[LeSteve123]

Hi

I'm looking find a suitable method to fix insulation to the walls in my garage conversion...

Outer leaf 100mm brick wall (inside painted with bitumen)
45mm Celotex (no cavity)
25mm x 47 battens at 600 centres
1000 gauge vapour control layer (polythene)
12.5mm plasterboard

(this gives the walls a 0.35 U value)

Building control are asking
"What is the proposed fixing? It should be similar to those used in timberframe projects."

I'm also wondering what the fixing distances should be?
Regards
Steve