I have tried the 780's before and maybe I just had a bad pair but within a few months the wire had tangled and curled and I guess it had pulled out of the headset and stopped working but the headset itself was great.
I hear very good reviews about the steelseries v2 but they are £20 more expensive than the other 2 headsets. The Sennheiser headset I'm not sure on that particular make but I heard they're a really good make.
Or if you have any other good headsets around the £40-50 mark, please let me know .
Never heard of Plantronics.
I know Steelseries makes great quality products (never had experience with their audio).
I know Senngeiser is a premeir, top-notch audio brand. Not sure about that particular model though. I would lean towards those just because of the things I've heard about the company.
But compare reviews and prices! It is ultimately up to your tastes!