An observation...
I use to think polystyrene coving was the devil's threadbare underpants.
Then I needed 6 3m lengths and couldn't face dragging a box like that from B&Q down to the bungalow, let alone putting it up (it's heavy and my car only takes 3m on the roof, precariously balances over the rails!).
So I got the Gyproc Lite 100. Same price, same profile as Gyproc 100 but 1/4 of the weight.
Turns out the polystyrene is quite dense with reinforcing tape. Went up a treat and (yesterday) painted beautifully. No visual difference with the rest of the coving at all. I'm a convert.
Cheers
Tim