Made my booking online but when I turned up to pick up the car as scheduled there was no one there. No way to contact anyone. Forced to make alternative plans.
Edit: I managed to speak with a representative (in the US) the following day who told me that the Victoria branch was closed for the weekend. They also told me that all customers had been instructed to collect from Battersea. The US representative that explained this situation to me was rude and dismissive and swore blind that all customers had been contacted. If I was reviewing Sixt corporate customer service they would be getting 0 stars for how difficult it was to actually speak with someone, and how I was spoken to during that conversation.
When arriving in Battersea the next day, I discovered that there was no collection scheduled and that my booking had indeed fallen through the cracks. Battersea branch rep was helpful and did find me a vehicle. I was also contacted by Sixt the following week and they were apologetic and promised to make amends for the situation.