Customer service severely lacking

In May I asked my financial advisor to increase my monthly RA premiums from June. I didn't notice on the quote that he actioned the date for May and not June, even though I verbally confirmed the actionable date with him. In June I was charged the new increased amount as well as the difference between my initial May payment and the new amount. On querying this my financial advisor informed me that because I had signed the quote, there was nothing they could do. I went directly to Liberty to query this further as I find it hard to believe that they cannot fix an error like this. If they had made the error and not me, would they still be making me liable for the nearly double payment? Or if the error on the quote was to add another 0 that no one picked up would I forever be liable to pay that amount instead of the amount I requested? Since the 14th Aug I have been trying to sort this out. I was told my query would be handled within 4 days, after a week I had to phone back as I'd heard nothing. Then my query would be handled within 48 hours, after 4 days I had to phone back as I'd heard nothing. Then customer service would have an answer for me by the end of the day. That was 2 weeks ago - I'm still waiting despite 3 further emails.