Hi Paul,
I too struggled with celeriac for many years and have had encouraging results of late – mainly due to planting in very compost rich soil and watering well. This is a link to some detailed instructions that may be helpful https://www.quickcrop.co.uk/learning/plant/celeriac
Also check Charles’ books for instructions.
Fundamentally, sow seed early indoors, plant out in compost rich soil, water well, remove outer leaves later in the season. Harvest and enjoy!
Good luck!
Beverley