interest rate changes on mortgage

hi

can you help me understand why my mortgage lender has put my mortgage payments up twice in past year ?
as i understand the bank of england variable rate is 3.99% for home owners. my lender bank of ireland is now charging 4.49% . my payments have gone up from £541 per month to over £800 interest only.
i have asked them and they say it is in line with the national rate??

is this correct ??

thanks. Michael