Double charged, unresponsive on email, cannot speak to anyone

I booked a car rental on this companies online website and the process was great. The problem is that I subsequently changed the number of days rental and at that time was immediately debited for the full (new period's) insurance. When I enquired about the refund for the old period I was told it can take up to 21 days if done by a third party. It is now 6 weeks later and despite a number of emails I have not received the refund. My last two email remain unanswered. There is no telephone number listed so only emails available to try and resolve a query