Sorry to hear that. Batteries earn £5-10 as scrap value depending in size! A mate of mine does scrap and batteries are one of he's best earners behind copper so id imagine they are sort after in the scrap world.
Prob pikeys feeling they have to steel and make someone elses life a little harder to earn there way The dirty thieving Bumhole scum they are!!!.
Why not run straight from van battery instead? Thats what i do and have done the last 2 years with no problems. No need for extra batteries then especally as you would need to go and spend £50 plus on them.