A shop assistant pointed out stiebel AQ range water heater has a plastic not copper water heater inside- what practical effect would this have on performance vs copper?

Pretty well zero IMHO.

So long as the heater materials are properly engineered I doubt you would see much difference in usable life.

Of course plastic does not conduct electricity, so there is potentially improved safety (the heater will still need an earth mind). That said, if a fault causes it to boil dry a plastic boiler will melt and potentially burn.

Never heard before, but indeed:

http://www.stiebeleltronasia.com/productdetail.php?productcateid=4&producttypeid=43&productseriesid=77&id=1114

"High efficiency new plastic tank".

They must have developed a safe mechanism to protect from overheating.

Never heard: its age old, I found reports from 2008

So it must be an approved technology.

it suprised me too, i wonder if it widespread in colder climates

i thought the thermal conductivity would have been impaired!

thanks for the information