Calculating interest

Rent Arrears

My tenant has continuously since August 2007 failed to pay his rent on time despite many letters reminding him that interest is due on late payments. The interest stipulated in his lease is 4% above base rate of the Royal Bank of Scotland. he is now leaving and `i need to know how
1) to find a table of RBS interest rates going back to 2007, and
2) as rates have changed so frequently will i have to calculate this month by month- or could I do it, say, in 6 monthly periods using the rate at the beginning of the period?
I can't imagine I am the only landlord with this problem?