Have searched but just can’t find what I need. Likely there’s a name for these so here goes …. 
 

I want to ‘screw’ 50mm celotex to the bottom of 150mm celotex.  This isn’t the only fixing. Where there’s a roof joist I’m going through the 50 into wood. But due to bad cutting / workmanship there’s a couple of areas where there is no joist to hit.
 

is there something like a 100mm big fat plastic wide thread screw that will hold celotex to celotex ?? 
 

once I batten below the 50mm it will be held better. A few of these screws would just be nice and fix a temp issue. 

Not quite what you mean as this is a plug, but at 85mm it may work: https://ewistore.co.uk/shop/external-wall-insulation/spiral-anchors/

1 hour ago, IanMcP said:

is there something like a 100mm big fat plastic wide thread screw that will hold celotex to celotex ?? 

No but if you peel back the foil on the stuff you want to cover and the 50mm layer that will face it, you can bond it on with low expansion foam. You can leave the foil on but tbh it sticks so much better if you get PIR to PIR 

I think an adhesive is probably easiest.

You can get non expanding PU resins.

But you may find that PVA works well.

I once bonded some polystyrene to timber with a urea-formaldehyde adhesive (Cascamite), then tested it for strength. The insulation gave way first.

 

 

Nice replies thanks. Looks like foam will do the trick maybe with a temp batten to hold it fast