Arrogant, unhelpful, and ignore standard accepted business practice

When I signed up I specifically amended the signed contract to the effect that I would pay levy "3 months prior to occupation". Tha amendment is contained in writing. In any normal business agreement, that means "90 days prior to occupation". Since my occupation date is 27 December, that means that I have agreed to pay the levy by 37 September. But LPA have already added a 'collection fee' to my account as since end -August. According to them, if my occupation date is ANY TIME DURING DECEMBER, then '3 months prior' means BY THE END OF AUGUST. That contradicts any accepted business practice, since it effectively (in my case) makes the levy due almost 120 days in advance of occupation instead of the agreed 90 days. When I queried this, their response was to send me multiple copies of the statement reflecting the disputed collection fee. I eventually managed to get hold of one of their call centre agents who promised to investigate and let me know the outcome, within half an hour. That was 3 hours ago.