Firstly get ICS to forecast your repayments based on the deal they offered you, at the current SVR rates. See how affordable this is. Then get a quote from other providers (with lower rates) for the new mortgage (assuming you pay off the existing one), the quote should include a forecast of repayments at current SVP rates. Ask them all to stress it for a +1% interest rate rise. Compare the outlay over time.
See what is affordable now, and what may be affordable in the future, factoring in children, possible pay rises etc.