You do realise that you're advising the OP to borrow money at 40% interest to save it at around 5% interest?
Where's the sense in that?
OP, if the interest on your savings is 5% (will probably be less than that), that's £20 pm interest tops, leaving you cost neutral.
However, you say your interest charges/interest/fees/however they dress them up are at least £20 pm, but could be as high as £200, so very very expensive.
Pay off your overdraft and start again with savings. Even if you only managed to save the interest you're not paying on your overdraft, you'll probably be able to put by £1k pa. Maybe reduce the authorised amount to £500 or less and pretend it doesn't exist, it's only there to make sure none of your DDs bounce.
You'd be a lot better off using a credit card to manage cashflow. If you put your normal essential spending on a credit card (eg groceries, petrol, travel, budgeted extras) and pay it off in full every month, it delays all this sort of spending from leaving your account for 3-7 weeks, so makes it much easier to cope with the ups and downs of income and expenses. This is completely free and you may even be able to make a small profit if you get a cashback or points card.