Grateful though I am for the wonders of underwire, I do NOT like it when it breaks and tries to poke a hole in a rather sensitive place. Ow.
Re: I've bodged it...
I think you can get new wires from places like Robert Sayles haberdashery department. Getting them to stay in place is the difficult bit.

As for sports bras, I bought one once and found it gave *less* support than the ones I was wearing anyway and only earned its name by being plain white cotton, no underwire, and having a little tennis racket motif. It will possibly be easier now I'm wearing a smaller size though.