Singing Pig

I'll cut a long story very short.

I've had a commercial property for the last 4 years and have a very good tenant in the ground floor. Flats above are let on AST.

 Initially I wasn't looking after the collecting of money (was looked after by my ex business partner) but have now decided that I need to look after the finances myself. It appears the commercial tenant was not charged for his portion of Buildings Insurance.

My questions are;

1 - Is it too late to claim back 4 years?
2 - What portion of the buildings insurance is paid by the commericial tenant?

3 - If i am eligible to claim, what is the best way to do it without causing any extreme embarrasment?

 
Many thanks.