I was thinking about this when I got the notification as well.
I’ve been assuming it’d be £4000 * 3% / 365.35 * 30 = £9.85 (or £10.18 for a month with 31 days), but I suspect the headline rate of 3% is actually APR even though that’s a bit disingenuous as almost nobody will be compounding it as most people will have a cash balance over £4000 at all times to take full advantage of that.
So, I guess we can assume that over the full year we will earn £9.72 / 30 * 365 = £118.26 and so effectively spend £119.88 - £118.26 = £1.62 on plus.