funambulist That is really frustrating. They should have realised that the job needed scaffolding when they gave you the quote. It is almost cheating to give you a quote ex-scaffolding, take a 40% payment, and then say they need to add something that includes scaffolding.
You might like to inform theme that you would also have to obtain alternative, scaffolding-inclusive quotations and that you would like the 40% returned in the meantime. [At least that would ensure that they are quick about getting a revised quotation to you].
Scaffolding is really expensive, the scaffold firm charges put-up, put-down and by the day it is in place. Your builders sound disorganised, so just make sure that if the scaffolding stays up longer than originally intended, any extra cost is taken by the builder.
I think the rest of your experience -pay, silence, chase, short-notice, etc. is very typical. I think it is hard for builders to do their scheduling. You do have to be on to them about exactly when they are coming, in a very nice calm way, of course.
But not realising that the job would need scaffolding rings alarm bells for me about their competence.