Understood, but any quote you get on the internet will be only be of limited use as local rates apply, as well as in your case how awkward the staircase is to do and what prep is needed. A plasterer will look at it and give you a price for the job which won't necessarily tie in with any metre rate due to the nature of the work.
So for example I would say it sounds like £400 would be my price.
A scrape, bond it, bond ,set, perhaps beads for window , soffit, external angle. Supply and fix. Removal of waste.
But that is of very limited use as you may get it done for less or be unable to get anyone to do it that cheaply .
So yes, look for internet quotes by all means but please don't set much store in them unless that person will actually come and do it for that price, and you can verify their work first.
People here including myself will give prices with the best of intentions, but we are only guessing and giving rough guidelines.
When spending real money on a real job, it's better to get real people to give real quotes!