Data just stops and you get a web page where you can buy more, the prices have gone down a few times. It was around £5 for 1GB for 7 days. If your month renews during that time you lose any data you've not used as its replaced by your standard allowance.
Edit:
I'm on the SIM only 10GB for £27.99 price plan myself, so I thought I'd check. The page at
http://add-on.ee.co.uk/purchase isn't working properly today and is only showing one option 100mb for £1.99 for 24hrs. (That web page only works over the EE network).
plusnet unlimited fibre - 2 Jun 14 - 470m - 80/20 - Summer/dry sync 55/9.4, Winter/wet sync 52/9.1
15 years broadband (1999 ntl:cable trial) - Asus RT-AC68U with HG612 - BQM - Summer PN speed - Winter PN speed
Edited by jchamier (Sun 04-Jan-15 12:28:43)