Hi
I did the same as Woodlands Camper, I took a piece of my grey waste pipe into the local builders merchant, Then matched it to the right pieces.
The whole thing cost £2.90 and as its all push fit as suggests took seconds to put together.
B&Q or other main brand DIY stores will stock the same fittings but its always cheaper to go direct to a plumber or builders merchant.
Hope this helps
John