300 Mbps? Hmm, I would have thought of Influxis (
http://www.influxis.com), but 300 Mbps will become quite pricey.
But since you're using a live stream, you might actually be able to utilise RPSes for this purpose. You use your current server to distribute the stream to a number of RPSes - say 1.5 Mbps MD stream each to about 3 RPSes (that's only 5 Mbps with the overhead).
These 3 RPSes will then stream it to multiple viewers. Given the 100 Mbps cap for each RPS, 3 would get you close to the number you're looking at (though outside of Europe your results will vary due to low priority peering on RPSes).
You should be able to use the lowest-cost RPS really, since you're not doing any encoding on these servers - just relaying what the main server already encoded. So about £30 extra a month, this would beat Influxis or other streaming media companies.
However, I would double check the T&Cs at OVH. I think there might actually be a clause regarding streaming media (high bandwidth). Not sure, but most other companies do.