[VeggieVirgin]

Does that mean it'll be no use and I'll need to re-sow?

It's been in the raised bed since last autumn, and has been producing a terrific crop since January.

[arugula]

I've never had it flower on me, but as that usually leads to things going to seed it doesn't sound too good.

[alibean]

perhaps you could save some seeds and start off anothet (free) crop for next year 

[RichardA]

nip out the flowering stalks it will leaf up again but keep on top of them. Best to sucession sow at about three week intervals. Same as for watercress.
R

[Nobbie]

Mine flowered last year and is still going strong, I also have the bonus of lot of seedlings coming up all round the original plant