That's what I thought too but although it disappears in the phone app after midnight (along with your old plan) all your carry-on data will reappear again once you buy a new plan.

Guys, I ran the numbers with Simyo and it turns out that working out a stable, "same tariff each month" type of approach is financially worse than buying a mammoth data plan in one month followed by 3 cheaper months. The reason? The 3-month data carry-over.

For example:
Person A pays €6.00/month for 18GB (+100 minutes).
4 months = €24.00 with a total of 72 GB of data.

Person B pays €12.50 in the first month for 80 GB (+100 mins) and €2.50 for the following 3 months (100 MB + 100 mins).
4 months = €20.00 with a total of 80.3 GB of data.

Of course if you really need a lot of data you could go for the biggest pack out there in the first month (200GB!):

€18.50 in the first month (200 GB + 100 mins) followed by 3 months of €2.50 (100 MB + 100 mins).
4 months = €26.00 with a total of 200.3 GB of data.

Just done this and worked as you suggested. However I was a bit surprised that the current plan really stopped as soon as you deactivate auto renewal and reverts to pure PAYG (minutes and MB charged at standard rates) so had a couple of days data charges but fortunately not too much!

Interesting. So all your gigs disappeared after switching off auto-renew... but then came back after buying a new plan? Technically it shouldn't do this as you are simply telling it not to auto-renew. A solution might be to switch it off just before midnight on the last day, right before purchasing a new plan.

I just renewed yesterday but I cannot remember if it pulled my plan after switching off auto-renew. I always do the procedure right before buying a new plan though (I couldn't bother waiting until midnight this time for the extra day's worth of validity), and I'm pretty sure that after the new purchase I saw both plans active (before midnight).