I went into a small store today to buy an unblocker. The woman in the shop said she thought this drain cleaner with caustic soda was the best. She could've sold me one at 6 quid, so she wasn't after a sale or anything. It cost me £1.25 for 500ml. I mixed about 100g with 500ml of water, then poured it down the drain. I don't think I left it long enough (about 2 hours) as it still hasn't cleared, but it is going down a lot faster. I'm going to let the drain dry then pour another 100g with water down and leave it overnight.