Right! There's also these impaling spikes or clips that you can use for that: And these things too: sort of like a nail with a huge plastic washer on it:The challenge is to affix the insulation to your inside out wall in such a way that it will not slip down over time - fishing line and some hooks or nails would do the job.
One thing to be aware of with the cavity insulation: if there's a chance your slab might get wet, then use mineral wool rather than fiberglass. Mineral wool holds up to water a lot better than fiberglass. Since part of Gareth's place will be below ground level, this might be worth considering.
- Stuart -