Here in Canada and the US, most of the time they just tile right over the water resistant plasterboard (which we call "greenboard" over here because it's green in colour) with a Type 1 mastic.
If it were me, I'd take the time to put something more waterproof over it. There's a water proofing membrane called "Redguard" that's very popular here. I've never used it, but from what I understand you just roll on two coats of it with a paint roller, and it dries to a waterproof film that both mastic and thin sets adhere to.
Here's a web site where a guy took pictures of his tub/shower remodel as the work is in progress and he used Redguard waterproofing membrane in a small area he tiled.
http://homepage.mac.com/advtrvlr/PhotoAlbum25.html
Maybe phone around to the places listed under Tile, Wholesale in your phone directory and find out if anyone carrys a comparable product locally and if it's appropriate to use over your water resistant plasterboard.
Where I live, the general concensus of opinion is that water resistant plasterboard isn't really all that much more water resistant than regular plasterboard. But, plasterboard made in different factories may be very different. Biggest hunk of people over here feel that water resistant plasterboard is waterproof enough to be a good choice for tiling a kitchen counter back splash, but it isn't water resistant enough for use in a shower, so if you can add protection with a waterproofing membrane like Redguard, I would.
I have used a product made by the Georgia Pacific company called Dens-Shield. It's the most water resistant product I've ever come across that's still made out of gypsum.
http://www.gp.com/build/product.aspx?pname=DensShield®+Tile+Backer&pid=4684&hierarchy=pc
When my sister had a mini-flood in here basement from a backed up floor drain, we had to replace the bottom 12 inches of plasterboard all around her finished basement. Instead of replacing plasterboard with plasterboard, I thought it'd be better if we replaced it with Dens-Shield, so that if that ever happened again, there'd be much less damage. I was given a piece of Dens-Shield to test, and I had it submerged in a bucket of water for about a week before suggesting to my sister that we use it. After a week of being underwater, only the edges of it were wet. If you scraped away the wet stuff on the edge down about 1/4 inch, it was still dry inside.
Since then, I've used Dens-Shield as the tile backer in several bathrooms I've done in my building. It's as easy to cut as drywall, and you put it up with ordinary drywall screws (or at least I do). It's prolly the most water resistant gypsum based board there is on the planet, and I much prefer it to the old heavy cement board tile backers like Wonderboard, which I've also used many times.
So, if you ever tile around another bathtub, keep Dens-Shield in mind. Not sure if Georgia Pacific markets it's products in the UK, tho.
