What type of pipe and fittings is this?
What type of pipe is this


Just plain 15mm plastic pipe for domestic water supplies. The elbow fitting looks like John Guest Speedfit, so the pipe may have been selected to match but I believe everything is pretty compatible nowadays.

See How to remove this plastic pipe elbow (scroll down for Speedfit)


  should the elbows be replaced, and if so with what / how?


I would replace the elbow and the short vertical section of pipe. Use the same type of connectors (for consistency, no other reason). I'd check the condition of the pipe stub from the wall and clean it up if necessary. 

See question linked above for how to remove elbow. Obviously you have to turn off the main stopcock and have some old towels or equivalent to soak up spillage. You may need to bung the loft tank ("attic" for leftpondians though you generally don't have this kind of arrangement AFAIK) or turn off a valve in the cold supply from the loft (otherwise you have to tie up the float valve that fills the tank and then drain the main loft tank which takes ages through a bath tap for example)


  How can I try to avoid twisting it in the future when I do this sort of work?


I usually clamp a mole-wrench (leftpondian: vise-grip) onto the body of the service valve and hold that firmly immobile with one hand while using a spanner in the other hand to loosen the nut at the top. Some service valve have flats on the body for this purpose but many don't. Take care not to crush the valve (I've never done this but it might be possible with a large mole-wrench and excessive enthusiasm)