Perhaps someone else can point out where I am wrong, but according to http://www.quinn-therm.com/wallinsulation/timbersteel-walls.cfm that will give you U=0.3ish which isn't even meeting building regs, which is itself worse-allowable practice. Might depend on your blockwork.
Should be aiming for more like 200mm imo unless space is an issue.
Should be aiming for more like 200mm imo unless space is an issue.