Hi,
I tried all option on charging and even used a normal 3 pin with a normal extension lead. Charging is however only 1.4KW so slow but works. Also you can set the charge box to 50% load so far I see on the manual.
The 5 pin cable charges at 4.8KW so faster, I used a 20m cable here again on 50% load (I see this on the charge box). Both worked fine. In Austria I even had to use a pin converter for charging as the 3 pin sockets are different from swiss to Austria.
I think if you use a normal extension lead (not the once on a roll) and the better quality (heavy duty once) should be fine, I had no issue with it.