hi
you could make your own extension lead
you need some 2 core 2.5mm cable which will be rated at approx 20 amps, if you have the ctek mxs5.0, which is 5 amp charger, the 2.5mm 2 core cable will be more than ample for the length your after, B&Q will do it or Ebay, 10m anout 15 quid, https://www.ebay.co.uk/itm/202862982192?hash=item2f3b937430:g:9C0AAOSwT1VfkerP
best way would be to cut the 12v lead and use blue crimp on, bullet connectors, something like https://www.ebay.co.uk/itm/174800182662?hash=item28b2e71d86:g:TbgAAOSwxDJcKLzf
cheers
Joe