Sixt car hire- Awful customer service!

I booked a 7 seater car from Sixt car hire, a Ford galaxy, to hire for this weekend as we are travelling to the Lake District for Christmas. We booked the car in October and paid in full. Yesterday (21st) we received a phone call to say the car wasn't available anymore as they had double booked them and they can only offer us a mini van! We told them that the van wasn't suitable for us and we need something similar to the Ford galaxy which we had hired and they argued that as it's a busy time of year they had nothing available! Disgusting considering this was booked in October!! After a day of worry and stress about how we would be able to go on our weekend away, we asked for our money back in which we were told we'd have to pay for the first 3 days of the booking, even though the fault was completely in their hands. Needless to say we are not paying this, we are seeking legal advice! We managed to book a car with Enterprise yesterday evening, they have been fantastic but sixt have been beyond atrocious, Everyone Avoid this company at all costs!