Four albums into their career, and The Unthanks are an act that seem to change constantly, yet remain resolutely familiar. They’ve changed names – from Rachel Unthank And The Winterset to the more representative shorter moniker – and they’ve changed their line up on numerous occasions. They’ve changed the music they dress their songs with – early on in their career they were draped with only sparse, wintery accompaniment, while now they’re bedded in lush, string-laden arrangements.

But at heart, this act has always been about two things: the voices of sisters Becky and Rachel Unthank, and an unrelenting sadness.

Last, the sisters' fourth album, defiantly comes from the same base ingredients, and as such sits comfortably alongside The Bairns or its predecessor Here’s The Tender Coming. So, if you haven’t heard The Unthanks yet, and their resolutely melancholic folk, why should you bother?

First of all, because of that 'f' word. This isn’t folk in that Coldplay with banjos sense. The Unthanks are musical archeologists rather than writers, preferring to put their own spin on songs they’ve dug up from local communities and half remembered rhymes. This does mean that there’s plenty of "lassies" and "me laddies" mentioned in their songs, and a preoccupation with death, men going off to war, mining disasters, booze, child death and poverty. If you’re put off by the thought of such subject matter, then it’s probably best to go back to Mumford and his pals.

If you do stick around, you’ll discover that The Unthanks' real skill is not in digging up these songs, but in imbibing them with a mood and a feeling that’s decidedly modern. This allows them to slip modern covers in almost unannounced, without them feeling out of place. Here on Last you find ‘Close The Coalhouse Door’, an epic exploration of the cruelty of a miner’s life, sat next to a cover of Tom Waits’ ‘No One Knows I’m Gone’. Neither feels out of place, and both sound like they could’ve been written at any point in the last 500 years.

What this consistency of mood does mean though, is that as a listener you’ve got to be in the right frame of mind for The Unthanks. Last is from the lusher, more beatific end of their scale (no clog solos), but apart from the title track it’s still difficult to sit through if you’re not in the mood. Listen to it on your way to work in the morning and you might find yourself in tears before you’ve logged onto your emails. Put it on at a dinner party, and your guests might be put off their pudding by the way ‘The Gallowgate Lad’s tale of wartime romance turns tragic.

If you can find that mood in which to enjoy The Unthanks though, it’s impossible to avoid being captivated by those voices. Both Rachel’s more precise tones and Becky’s breathy vocals get equal prominence on this LP, the contrasts between them and the sheer harmonic Northumberland beauty when they come together rendering the lyrical content moot on many occasions. They could sing the songs of Las Ketchup and make them sound impossibly sad and beautiful.

So yes, Last is pretty similar to their previous albums, and there’s no real sonic step forward from The Unthanks, but there’s a timeless quality to the band that suggests they’re not concerned in the slightest. In some form or other they’re going to around for a long while yet, much like the songs that they love to unearth and make their own.