tomato plants

I think it's likely to be over-watering or irregular watering - although, what you can do with outside plants when it tips down I don't know! Are the plants which are OK in a slightly more rain-sheltered spot than the one which has problems? The skin splits when the inside of the fruit swells faster than the flexibility of the skin can keep up with, hence regular and 'sensible' watering to keep the two in balance. Some authorities also blame over-feeding which can have a similar effect on the innards, growing faster than the skin. Not a lot you can do once they've split, though - Soup??

Yes many of mine are now splitting which I think is due to all the rain. I skin mine in boiling water and freeze for cooking.

The few i'm growing outside are splitting, but indoor one's are fine.

Yes, think it's the rain, the inside is growing faster than the skin.