I have a 6ins peanut plant growing in a pot on a sunny window sill I have no idea what to expect or how to care for it we live in Cornwall and have a garden and a greenhouse

The important thing with peanuts is to plant them in a wider pot than you would expect, with nice soft soil surface, because after flowering they bend their flowered stems back down to the ground and bury the tips, and then form the beans under the soil (hence "groundnut"). They are grown in places with very hot and humid summers, and lots of light, so that is the conditions you need, maybe in your greenhouse, then, when it warms up.

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The soil should be loose, light and sandy, with plenty of organic matter like compost and well rotted manure.

Rocks and sticks should be removed.

A soil pH test should be done to determine the lime and fertilizer needs for growing peanuts in your soil.

A soil pH of about 6.0 would be optimum for peanuts, but the crop can produce well over a wide range soil pH.

If lime is needed, broadcast the recommended amount and incorporate with the soil well in advance of planting.

Mix in ½ cup of 8-8-8 fertilizer for each 10 feet of row prior to planting,
however if the area was heavily fertilized during the previous year, you can use considerably less.