Thanks Keith. I had already looked in the Manage My Booking section, and the extra case will be £28 for the return journey for 20 kgs. I didn't want to click and pay, in case "extra charges" were suddenly added on for amending our booking. £28 is enough to pay anyway.
We had only booked one case between the 3 of us, due to Easyjet's hand luggage not having to be weighed. However, giving it some thought, our daughter will probably use most of the 20 kgs allowance, and I've got all sorts of things to bring over for our apartment!