celeriac has bolted

You did everything right, but sadly it's the weather to blame. Celeriac, and relative celery, cannot take great fluctuations in weather as we've had. And cold weather tends to make celeriac/celery bolt. So I assume you've had a length of cold weather.

It's really hard to say whether or not it will continue to bulb up. You've cut off the flower head so let it grow a bit longer and see what it does. If it continues to put out more flower shoots, then pull it up because it will continue to try to seed and not bulb. And keep your fingers crossed for more stable weather next year.

This is my first year growing celeriac, its in the corner of my poly tunnel. Sorry don't know the answer to your question but just thought i would add i am  growing it to hahaha.

Buy some celariac plants from a garden centre.

Hmm I already have loads of french beans on the go, climbing and french, but I might see if I can find some celeriac plants in the garden centre, that's a good idea.