Mobile phone contract prices can often be obnoxiously sky-high, especially if you are subsidising the cost of a premium smartphone. But if you already own your phone outright, there is a much cheaper way to pay for mobile service if you shop around for a SIM only plan.

SIM only plans do what they say on the tin, offering you a SIM card with minutes, texts and data for a monthly price. While you can get these from the UK’s four main mobile networks EE, O2, Three and Vodafone, SIM only plans are often cheaper if you go with a mobile virtual network operator (MVNO).

MVNOs lease use of the four main mobile networks to offer lower prices to UK customers. Familiar brands include Lebara, Tesco Mobile, Sky Mobile and Giffgaff. This week, it’s Giffgaff that is running an eye-catching SIM only deal that triples the monthly mobile data you’ll get if you sign up to a new 18-month plan. Giffgaff uses O2’s national network.

For just £10 per month, you can get unlimited calls and texts with 75GB 5G mobile data. This price usually only offers 25GB. Giffgaff says if you sign up, you’ll get 75GB for the full 18 months, only dropping back down to 25GB on the 19th month.

75GB is more than enough mobile data for most people. You’ll only need more if you stream loads of video and never connect to Wi-Fi, but if you have Wi-Fi at home, it’ll very likely suffice.

If you think you do need more, Giffgaff is offering 150GB for £12 per month or a whopping 300GB for £15 per month, both on 18-month contracts.

View Giffgaff’s 18-month SIM only deals

It’s one of the simplest and best SIM only deals going right now. If you would prefer not to commit to 18 months, you can opt to use Giffgaff on a monthly rolling data plan instead. This means you have the flexibility to leave at any time, or even switch your plan month to month if you need more or less data.

View Giffgaff’s monthly rolling SIM only deals

If you sign up before May 18, you will get 40GB data for £10 per month, double the usual £20. Unlike the 18 month contract, this double data will only last for six months before returning back down to 20GB. Still, that’s something for nothing.

If you need more data on a flexible monthly rolling plan, £15 gets you 150GB, triple the usual 50GB, or you can pay £20 per month and get 300GB, triple the usual 100GB. As with the £10 plan, these triple data deals will last six months from the time you sign up, before dropping back down to their usual data allowances from month seven.

If you want to pay less than you currently do for your mobile plan, you can switch to Giffgaff fairly easily, especially if you are not tied to a contract. If you want to keep your number, it’s easy to do so by notifying your current provider that you’re leaving and want to take it with you. You can read Giffgaff’s full instructions on how to port your number to your Giffgaff SIM card here.