why are my tomato plant leaves curling under

Tomatoes are very vulnerable to temperature fluctuations. Could they have got chilled overnight? Has gone a bit chilly in the last few days, here in East Anglia anyway. If you cannot see anything curled up inside the leaves, this would be my guess as to the cause of your problem. It wont harm your crop.

I believe it can also happen when tomato plants are growing very strongly and healthily

i'm in east yorkshire and mine are curling too. they always do and i have always thought it was due to the high day time temp and cool night temp.
welcome to GoY too.