It could be chlorosis. It's a nutrient deficiency. Most common causes is poor drainage, too much water, compacted roots. A picture would be helpful

for this time of the year its not good if yellow at the top I would be worried it was the early start of blight

How are you growing them greenhouse ?

as Bathgate has mentioned already , feeding ? watering etc....

Gg

Sorry I did try to send a picture for some reason it didn't work ,had blight in the past ,don't look like that other tomato's near are ok will try to do photo again thanks

Fluctuating temperatures maybe, could you be overwatering especially lade in the day?

Cold nights and wet roots may cause them to rot
It is unseasonably chilly at the moment