Damm (down by the river in Amposta) has Glyphosate or Glyfosato in Spanish (that's the generic name and there are lots of brands) at 18E/5 litres at the moment which is the cheapest I've seen it in five years,  It's the same as Roundup but a lot cheaper. 200ml/cc per 16 litres is about right but if you haven't done it before I'd suggest it's heavy to lug around.  I find that 10L at a time is manageable so use 120ml/cc in 10 litres of water and add a small dash of washing up liquid (helps to wet the leaves).