Zest service was above and beyond.
Picking up my hire car at Santander airport, I was asked to pay an additional small fee of 50cents/day 'environmental contribution'. I queried it with the company (Keddy/Europcar) and was told everyone pays it. It was only 5.45 euro, so I paid it.
On return to UK, I submitted a review in which this small extra charge was mentioned.
Zest read it, then contacted me, confirmed I had already paid at time of booking and investigated on my behalf. They secured me a refund from Europcar without the need for me to do anything further. Excellent service indeed.
Incidentally car was excellent (brand new!), pick up and drop off also very good.