1.6td jx losing water

[SEKN410]

hi all, have checked the wiki so apologies in advance if i missed it. i noticed my top up tank was v low so topped it up to the max before a 150 mile round trip to tywyn. re checked on arrival and again when i got home and it was about a quater of an inch below the max again.idont know if this anything to worry about but im off to france thursday morning for the rugby. a friend suggested some leak stop but it says put it in the radiator. do i put it in the top up tank or header. is there anything else i should be checking. thanx all eugene and tracy