Primrose wrote: I'll probably use this year's leftover seed next year as it seems extravagant to buy new packets every year.
I would buy new seed next year since celery and celeriac seed does not keep.
A long growing season is key with celeriac. But it is also very much a "field crop", and does not do anything like as well in the garden or allotment as it does in a farmer's field. I have never been certain of the reasons for this, but it is true of certain crops: swedes, for instance, grow bigger for the farmer than the gardener. I have never grow one bigger than a grapefruit, but that is big enough for a good serving for several people, rather than cutting one in two (the reserved half seldom keeps well). Celeriac needs full sun, which is probably not what it has been getting growing near a box hedge in mumbling Monty's garden. I haven't seen his, but I expect the foliage has got a little drawn towards the light. Usually it is stout and stocky, regardless of variety. And a heavy soil; it does prefer a heavy soil.