I have 2 children both under the age of four I get my children at weekends but recently the mother of my children has been arguing with me a lot about when I can see my children so I woz wondering what are the chances of me gaining full custody if I applied for it?

Slim.

Unless there are compelling reasons or mutual consent, courts are reluctant to change custody.

Are your visits court ordered? If so, don't argue with her about them... its not up for debate. If they are not, get them ordered.