Jesskle, I'm in the States, where we have a different procedure, but I wanted to offer words of support and encouragement. I'm sorry this is so upsetting and of course you are distraught, given what you've described.
Definitely have a nurse or other staff member there with you. They can be a great distraction as well as support (and some people report their family members behave better with non-family around). Also have someone there to support YOU. And if possible, do something nice for yourself afterwards or later that day (chocolate in the car counts if there's no time for anything else).
Otherwise I wish you luck, and creative thinking. I like canary's story of how she told her mum she was making it legal to pay the bills! When my mother starts to yell at me about "what have you done with my house" I calmly reply that I have cleaned out the refrigerator so the food wouldn't spoil. Inevitably she replies, oh, good idea and then I distract her. (And I did clean out the fridge...along with the closets, the living room, the bedrooms, the bathrooms and we're still cleaning it all out!)
I hope you find an approach that works for you. Good luck!