I was calculating on what my spread charges me. The two of them charge £450 a day labour (together) and I reckon this job was 4 - 5 days. Don't forget the clearing up on a job like this, all wall splashes have to be removed (or walls protected) etc. Not as straightforward as a new build.
This may seem high but that's what we have to pay around these here parts
I had a customer on the phone a while back trying to get quotes for a new kitchen floor screed (new extension). Room was 12' x 12'. Best labour only price she'd had was £800! Not bad for half a day!
I also find that trades such as plasterers see a 'one-off' job such as this, as a chance to earn a few bob as they're never likely to get any more work there. They price work differently for builders as they want repeat work. Might seem unfair but that's how it is.