Was looking for a decent plunger and came across this at Screwfix for £11.99 as I am a regular shopper there. Then decided to look around and found it at a very reasonable price of £3.95 at Toolstation, it's going for £5.28 - £10.76 on Amazon.
When I got it I didn't realise it was made fully of plastic and not rubber and it was huge, but for under £4 it was worth a punt. Worked a treat due to the size of it creating lots of pressure in and out.
I used soda crystals and vinegar/chemical unblockers to no avail but a quick blast of this did the trick.