These refurb Sennheiser HD560S are a steal for under £100 from Sennheiser
A fantastic pair of open back cans.
Some of Sennheiser's open back headphones have gained legendary status for offering excellent audio and comfort from a brand who've been in the game for an awful long time. Some of their HD5XX and HD6XX releases come to mind in particular, including these HD560S, which can be yours for £96 from Sennheiser directly.
Before I go any further, I should say that these are a refurbished model. Sennheiser's own terms dictates that their refurb items are returned products that have been tested rigorously so they are up to standard, and they come with all accessories and cables where applicable. Customer reviews of these refurbished models in the past come highly recommended and virtually as-new. I've purchased from them before, albeit many years ago, and the headphones were as-new.
The HD560S look similar to some of Sennheiser's other cans within the same lineup, offering a sturdy plastic frame and a comfortable fit. A 240g weight keeps them especially lightweight, while there is also a lot of padding and a reasonable clamping force. This is a far cry from some of Sennheiser's other cans, which come with quite a heavy clamping force - I'm looking at you, HD660S2. They also have a detachable cable for convenience, although with a 120ohm impedance, you will want to be using an external USB DAC or amplifier with these to get the most out of them.
These are open back cans, meaning what they lack in a more robust low end, they make up for with excellent stereo imaging and soundstage. However, with that open back chassis, they are only really suitable for home listening, unless you want those around you to also indulge in whichever tunes you're listening to. They offer an excellent neutral sond profile too, with great detail - this means that no specific part of the mix should be too overbearing, or have too much prominence over other frequencies, while presenting rich vocals and instrumentation across a variety of genres.
If you're after a fantastic set of open back headphones that are lightweight, comfortable and offer great all-round audio, these Sennheiser HD560S from Sennheiser directly for under £100 are a steal.