Cable - yellow or blue

We have been told to get a long cable to plug in our Motorhome to the outdoor socket to keep leisure battery topped up when MH is not being used. (It seems that the alarm will drain the battery over winter??)

We have bought a mains coupler to plug into the MH - do we need blue 1.5 mm cable or yellow 2.5mm. I have read that one is 16amp and the other is 25amp. 
Can’t seem to get hold of one in our local camping stores.