A couple if ideas, Spacer............
"Masterboard" or similar brand. Inert and unaffected by damp. Quite expensive, and usually used in fireproofing situations. Various thicknesses including 6mm (may even be 5.5mm)
Also... cement fibre soffit board (usually used as the flat bit on the underside of your roof...... between the fascia board and the wall.)
http://www.roofingsuperstore.co.uk/product/pitched-roofing/pitched-roofing-1/non-asbestos-undercloaking-strips-1200-x150-pack-of-10.html
Various thicknesses again, including down to 3.2mm. The widest I've come across is 300mm. You may have to use several.
Both ideas are available from any Builders' Merchants.
Glue to wall if it's a cavity construction. If a solid wall and you're not sure about damp, then just nail or screw.
Also, as per Bright Spark's post................. a 6mm version..........
http://www.travisperkins.co.uk/p/hardibacker-250-tile-backing-board-1200-x-800-x-6mm/682191/3893473