My mum just died and I bought her house but one of the sinks was slow to drain. My brother in law said he would unblock it and poured caustic soda crystals down the plughole. This seems to have set and now the sink is completely blocked. Is there any way to dissolve the caustic soda that has set? I was going to try boiling water but my brother in law has poured a bottle of vinegar down the plughole instead. Nothing is happening - but the room stinks of vinegar. I would call a plumber but the mortgage payments are taking up all my wages! Any advice would be greatly appreciated!