Glysophate in too stronger solutions is more ineffective as it is less able to translocate. Its not only in Monsanta's 'Roundup' and not an exclusive chemical to Monsanta. Eg. Clinic Ace is an example of another brand. You can but stronger concentrations online like Roundup ProBiactive etc but you should be certified.

You could also try Ammonium sulphamate for stubborn weeds, its another translocated chemical that was banned by the EC for no particular reason but is still available as a compost accelerator.

To be honest I think Monsanta's lobbying of the EC is about as strong as it is in the US which is very sad considering 99% of our UK laws come straight from Brussels.