After multiple emails, Sixt have now refunded all of the cleaning charge. Persistance appears to be the key.

Same for me. Hired a car from Montpellier railway station - collected out of hours on a Saturday evening and dropped key 15 days later on a Sunday morning - all as agreed. When invoice came through there was 83 Euro plus 20% tax additional cleaning charge. Apparently sand on the carpets after 15 days at the seaside with two children falls out with normal between hire cleaning. Not a cheap hire at 1000 Euro for auto BMW 320 GT, could at least include basic cleaning. Sixt have rebated 50% of the charge (but not 50% of the tax ...) have now passed the dispute to American Express on the basis that I had not consented to the charge. Watch this space.