I have to say, welshlass181, if it were me, based on your recounting of events, I would not be paying the £159 but giving the vet the opportunity to have another go at producing an acceptable bill.
You were quoted £60-£80 and as far as I can see from your story you were not asked about the ketamine, so they decided to do that - and double the bill - without discussion with you. Fair enough, you'll have signed an authorisation and they must've had a reason. Did you have a mobile with you? Could they have contacted you to confirm that you wished to go ahead with all three being anaesthetised? (You might have decided to butch one or more instead?)
If they could and/or should've discussed it with you then I would ask if there is any room to reduce the bill.
Our vet will normally, if asked, make a reduction if there is a mix-up like this.