Need help with laws and footballs and bastard neighbours

[thfcmad]

OK basically we have an old witch that lives next door. Me and my ******* play football in our garden and our ball keeps going over the fence. Now we never go over and get it and just wait for her to throw it back over. Recently she has stopped throwing them over and has now started taking them indoors and keeping them for weeks before throwing them over. Basically what i want to know is what are the laws regarding going to get the balls back. This women is a real witch and would prosecute us if we gave her the chance. If i start climbing the fence and getting them what can she do?? I know she cant legally take the balls in as that is classed as "theft by finding" and i think she legally has to throw them back. Can anyone clarify what we can do so this doesnt get out of hand, is there somthing i can threaten her with so that she has to throw them back? Thanks in advance for your help.