If you have the misfortune of being on O2 like me, it might be worth checking your account on the app.
I ran out of data this month, as I quite often do, and went onto the app to see if there is a one-off top up available to see me through.
While I was looking, it asked me if I wanted to change to a higher allowance tariff, and while I was scrolling through these, I noticed that the monthly payment for 6GB was the same as my current 3GB.
This is probably linked to the fact they have a double promotion on new contracts too. It didn't mention how long it lasts for (if it's just a promotion).
This doesn't extend your current contract (thank God) and kicks in on the first of the month.
The image doesn't show the allowance, but it will cover me if they try to lock me in or charge me more!
So basically I'm now paying £32 for 6GB unlimited / unlimited instead of 3GB unlimited / unlimited (edit. which includes my phone)
Edit. I should probably add this contract cost includes my Galaxy S7 edge which I got on release last year! Not sim only!!