What's happening to my turnips?

I planted turnip over two months ago in trays. Initially outside but they were later transferred to a heated greenhouse following advice from helpful people on here. I can barely see bulbs on just a few of the plants so I’m not too sure what is going on. I hope that they all have large bulbs under the surface but would be surprised to find this has happened.
I always assumed that the turnips will be ready to be harvested when the leaves die, as they do with potato plants, but they show no signs of doing this. Youtube videos show turnips being harvested will all the leaves looking a healthy green.
I’m newish to growing veg. and I’m just getting more and more confused about what, if anything, to do. Any advice gratefully received.
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I imagine they're confused with Spring-like temperatures but mid-winter day lengths and extremely weak light.
I'd have thought with a sowing in Sept, you'd overwinter them in a cold frame (or somewhere sheltered) then plant out in Spring when they should be ready to harvest in a few weeks.
It's not something I do, so just my thoughts
Billericay - Essex
Knowledge is knowing that a tomato is a fruit.
Wisdom is not putting it in a fruit salad.
I sow about 6 lettuce seeds in a small pot every 3ish weeks from March-June and that gives me plenty of lettuce throughout the growing season (depending on the weather) from a very small area.
A friend of mine planted some chitted spuds a few weeks ago hoping to have them ready for Christmas lunch. He says they're growing quite well atm.
See how they come on over winter. They will start to grow quickly as spring gets underway.
Billericay - Essex
Knowledge is knowing that a tomato is a fruit.
Wisdom is not putting it in a fruit salad.
Hi Pete
The turnips (and some leeks) were purchased from here:
https://propagationplace.co.uk/buy-vegetable-plants-online/
They advised that these were ready for planting at the end of August/beginning of September, but we are up in the Moorlands, so it depends. The plants are fine and I've had plants from them before.
I note that you are in Essex where a cousin of mine moved to many years ago. She visits once a year but says that she could never come back to live here as it is so cold and wet. At around 4.00pm today it was 2C outside and we have had torrential rainfall and a cold wind. This, no doubt, explains the state of the turnips and leeks (outside in tubs). I'll just let them do their own thing and hopefully they will be ready for harvesting before I plant the potatoes around Easter.
The only other thing outside is the garlic. This is sheltered as they do not like too much water. We planted them from garlic bought from Lidl, not the recommended way to do this, but they are doing fine.
They won't grow much in the low temperatures but they should be OK
I sowed beetroot at the and of July and harvested golf-ball size beets in Oct.
I made 6 litres of beetroot and feta soup for the freezer - served ice cold with a glug of Polish Zubrowka vodka for warmth - very refreshing
Sounds like your garlic is happy - maybe turnips sauteed with garlic next year..... or maybe not
Billericay - Essex
Knowledge is knowing that a tomato is a fruit.
Wisdom is not putting it in a fruit salad.
You will be far better off buy seeds and growing root veg that way.