You might have to fix your repayment amount at a particular amount and increase it to compensate if there is an interest rate increase or you could fix it at an amount that will repay the loan a bit early if there's no further interest rate increases (or more quickly if they start coming down). I did something similar with my AIB mortgage.
Even with a variable rate mortgage? Not sure I follow you here entirely.
so now a second question, does the lump sum payment have to shorten the life of the mortgage and keep the monthly payments the same, or can you reduce the size of the monthly payments and keep the same 30 year term?