Since its launch earlier this year, Microsoft's Lumia 640 has proven to be a big hit, especially over in the US, where just three months after it went on sale, almost 10% of all Windows Phones in use were 640s. It's done well in the UK too, where it's already among the most popular Windows Phones ever sold.

As we discovered in our recent review of the Lumia 640, there's a lot to like about it - from that big beautiful display and competent camera, to the solid performance and superb battery - and it's even more appealing given its low price. For UK buyers, that price is now lower than ever, especially for pay-as-you-go (PAYG) users.

The Microsoft Store sells the Lumia 640 SIM-free (i.e. at full price, with no carrier subsidies) for £129.99, and leading independent retailer Carphone Warehouse does the same. But the retailer also offers the device on monthly carrier contracts, and on PAYG, for those who prefer not to be tied down.

If you're a PAYG user, Carphone Warehouse is currently offering the Lumia 640 for just £69.99 as an upgrade. PAYG upgrades are only offered on O2, Vodafone and Virgin Media, and as the retailer explains: "It must have been more than 6 months since you purchased your phone or last upgraded on your network."

The price you pay to upgrade to the Lumia 640 will depend on your network:

Virgin Media Pay As You Go Upgrade: £69.99

Pay As You Go Upgrade: Vodafone Pay As You Go Upgrade: £79.99

Pay As You Go Upgrade: O2 Pay & Go Upgrade: £89.99

If you don't qualify for an upgrade, you can still get the Lumia 640 for less than you'd pay for the SIM-free handset. Carphone Warehouse is offering the 640 for £99.99 with a minimum £15 PAYG top-up (under 'Pay as you go (New SIM)'); but even though the SIM offered at that price is from EE, almost all of the handsets that the retailer sells are network-unlocked, even on PAYG.

The £114.99 total payable for that deal will save you £15 compared with the SIM-free price - and you'll be able to use it on the network of your choice, with integrated 4G LTE connectivity.

Source: Carphone Warehouse - Lumia 640 on pay-as-you-go / Lumia 640 SIM-free

Neowin global giveaway: Microsoft Lumia 640 LTE

Don't forget: this week, we're giving away a Microsoft Lumia 640 LTE, along with a Nokia MD-12 Portable Wireless Speaker, to one lucky reader! The contest ends this weekend - but there's still time to enter, so head over to the giveaway page... and good luck!