Yes it needs to be stripped. If you have paid the plasterer to strip it then he will. At plasterers rates it will be much cheaper to do it yourself. If you want him to do it then make sure it's agreed and quoted in advance.
Excellent. Next question, I see you can get an attachment for a karcher steam cleaner. Has anyone attempted using just the normal floor cleaning head? The proper attachment is £30 (I know about steam blowing the plaster but time is of the essence and the thought of scraping it off makes me die a bit inside)
Most wallpapers will strip easily with hot water with a little bit of washing up liquid, leave it for a couple of minutes, I prefer it to steaming, (unless you're talking wood chip which is a pain to get off