I took my own stuff Linda but In a Nathan neoprene belt which fitted nicely into the small of my back - I managed to fit gels, jelly beans, blister plasters, tissues and mobile on it. I wouldn't fancy carrying a case for 26 miles but if you trained with it you may be ok.
I ran my first marathon with a water bottle containing lucozade mixed half and half with water but last time I did without it because it was cumbersome after a while and there were drinks stations every mile (and to be honest I couldnt face another sip of it after about 17 miles!)
Tips would be
Having a decorators suit and or bin bag for the start
Covering everything in body glide or vaseline
Not wearing, eating or doing anything new on the day
Starting slower than you want to or feel like
Savouring every moment - it is a massive achievement and when you look back on it you will think time flew by (though perhaps not at the time!)
Hope you really enjoy it