Decided it was time to change the plugs ( twin ) as they had give me some hassle last time I took caravan out did the standard black one no probs did took gray plug apart to find no blue wire for battery charging also one of the earths missing what do I need to fit so battery will charge while towing

Hi mate, Have you traced the wires from the plugs back inside the vehicle? Unless its a manufacturer specific "plug-in" kit it will have been hard wired in to the vehicle electrical system. There should be a relay at the end of the wiring from the grey plug - this switches the signals from the reverse light, battery charging and power. There should be a power cable wired in to the fuse box on a 20A min fuse or direct to the battery with a 20A inline fuse fitted which supplies the power to the relay, which then switches the power for battery charging via the blue wire.

hope that all makes sense!

You probably have the std equip towbar electrics which do not include any power to the towed vehicle.

Will need to buy a kit or make up yourself