Hi Robin
Interesting, I think you have been drier than here? Usually that is a reason for small celeriac, but leaf miner must be hurting too (is it also on flat leaved parsley and celery?) and I don’t know how to prevent that. I had it last year but not this year, and my celeriac have swollen nicely, though not as big as last year. I would water now because soil is dry; I think celeriac bulbs up a lot in autumn, once its root system has established.