@Josh55
Personally If someone came to me with your problem now id do one of 2 things. I'd agree a price and schedule of works and then I'd do the following.
1) I'd wait till next year when weather is dry and better again. Then I'd hack it all off and start from scratch doing it properly.
2) I'd hack it all off asap and then possibly get a scratch coat on and then leave till next year to finish off.
Last thing you want to do is try and rush it and it goes wrong again.
I can't see you finding someone decent who will be able to squeeze that in before Xmas. I def couldn't, all my exterior jobs I've already booked in for March onwards.
The weather is going to get a lot wetter and colder in the next few weeks as winter is coming.
If you try and rush it you risk it failing or going wrong again.
Also you really need to come to an agreement with whoever done it and recoup all or as much is realistically possible?
Ideally you want money back from them, if you agree for them to put scaffold back up and hack off the existing then your again at there mercy. They could easily mess that up!
I'd advise against going over what's there as if it's as bad as it looks any guarantee from a new Renderer will only stretch to what they do. If the existing comes off or gets worse it will spoil the new finish so your back to square one with more money down the drain.
The idiots who done it have already gone over most of what was on there before anyway so that weight aswell as another coat isn't a great idea.
How you go about sorting the problem from here will determine the lifespan of the correctly applied new finish. Do it right and it will cost you another considerable outlay but you won't have to touch it again.
Do it wrong and try to save money now and it will cost you again in the near future.
Best of luck I hope it goes well from now.