Hope it's helpful , but have you tried one of the emergency call out window repair people ? they sometimes carry the stuff for late night repairs to shop windows
If it's just a flat piece you need, try the Glass & Mirror Center, Kingston Rd in Portsmouth.
I've had flat perspex from this guy before & he's very helpful. He even hand cut a small round piece of glass for me when the front plate of my Walker Log got cracked.
Talking Plastics advertise at back of the mags every month and are at Horndean. There's another at Waterlooville who advertises from time to time who's name I can't recall.