Looked this up for you - it is a biennial or short lived perennial so if nobody has any better advice I would cut off just the flower spike and hope for the best. Or you could wait for the seeds to ripen and have a go at growing some next year.

They do not usually survive after flowering as they are biennial Grandma. Have a look round a good GC and check if they have any small plants to take its place. Thats what I used to do til my border got so crowded I couldn't grow them on anymore. They are 'wow factor' in a small garden too.

Thanks very much, it was lovely to have my own olympic torch!