Just digging around to check this company is not a scam - it sells at a very favourable wholesale price (but only at large volumes i.e. multiples of 80 sheets, which may be the reason why they're not really a direct competitor to most stockists).

 

https://buildwholesale.co.uk/shop/building-supplies/insulation/internal-wall-insulation-insulation/partition-wall-internal-wall-insulation-insulation/rockwool-rw-slabs-partition-wall-internal-wall-insulation-insulation/rockwool-rwa45-100mm-x-600mm-x-1200mm/

 

Has anyone here used / heard of them?

They don't seem much cheaper than e.g. https://condell-ltd.com/rockwool-rwa45-600x1200x100mm-4-per-pack-2-88m-20-per-pallet

50 minutes ago, Mr Punter said:

They don't seem much cheaper than e.g. https://condell-ltd.com/rockwool-rwa45-600x1200x100mm-4-per-pack-2-88m-20-per-pallet

@Mr Punter Thanks - our starting point had been this builders merchants which appears to advertise each slab as being £35 vs £7, but your link highlighted that it may well be a pack of 4 (but I've re-checked the website and this one definitely reads as being £35 per slab (which would be ridiculously expensive!)

https://www.coversmerchants.co.uk/rockwool-rwa45-insulation-slab-1200-x-600-x-100mm

It is always worth phoning if you need a decent amount.  Bear in mind delivery costs can be high on insulation.