It is money I used to put against the mortgage. As long as I'm getting at least my mortgage interest rate on my savings then it's a no-cost option. I am n o worse off for saving it and I can always lump it against the mortgage if rates drop further, but it's nice to have it in cash in the meantime. Agree as a pure rate arbitrage it's not worth it and it's administratively simple, just a bunch of standing orders from my main current account. I see it more as cost-free option for the cash rather than locking it up in mortgage repayments.