Allotments 4 All

It's the first year I've got the timing right and I've got a nice little bed of them hurtling towards maturity.

My question is can they go too far?  Should I crop them now when they've just got some tiny hearts?  I thought one of the them tasted ever so slightly bitter - is that because they've gone too far or it's too dry and sunny or what?

Also, I read somewhere that if you could cut them in a certain way they keep growing  - anyone know anything about that?

Thanks...