Not in Cornwall but South Coast-you are going to plant in 3 weeks time??
Sounds incredibly early even for Cornwall-but if you have done it in the past......
I would cover the soil with a polythene structure now or cloches now to warm up and dry out the soil
I assume you are now chitting the seed potatoes?
One thing I have learnt over many years you can never garden to the calendar or predict the weather
I still wonder about planting so early as blight tends to strike maincrop and there is still plenty of time for harsh weather, frosts,snow................ 