You can get ducting to protect plastic water pipes, but as for the fittings, I'd try to keep them accessible if I were you.
If they are in a chase you could put a wooden lid on the chase so that the whole run is accessable.
Fittings in unaccessable areas are always a bad idea.
Just imagine you get a problem with one and end up having to dig up all around it, and theres no movement on the pipes cause they are solid in screed so it would be a pig of a job.
I'm sure that John Guest Speedfit make a plastic ducting to fit around plastic pipework.
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