Allotments 4 All

I acquired 18 strawberry plants at a car boot, for £3, they are about an inch and half across with about 6-8 leaves.

Question is what to do with them? Stick them into pots, and overwinter them in the greenhouse till next year? Plant them in the ground now ?

If you have spare ground prepared plant them..they over winter much better in ground..and they've got all summer to get their 'feet' in and give you full crop next summer. Who knows..you might get some already this summer.. ;)

They sound like seedlings to me.    Any idea of the variety.   I would bung them in the garden.    Very hardy things strawberries.  Are they perhaps alpine strawberries?

Heres a piccy, seemed a good buy for £3   8)

If I were you, I would stick them in bigger pots and leave them outside to enjoy the summer.
Then if you can get them in the ground next year so you can get the full benefit from the plant.
I kept mine in pots over winter on one of my deep bed edges and then I put them in the bed the following year. They are now in fruit and will hopefully give a good crop for us to enjoy.

They are good buy..and being so small plants, I agree with Digeroo,  they could well be alpine strawberries in which case they will fruit some for you this year as they reach maturity much quicker and will crop until frost anyway. They are very hardy..and as soon as the plants have started fill the pots with roots get them in.. ;)
Alpine as excellent 'gap fillers' you can stick them in all those little pockets of ground where you would not bother plant any veg..or even use as edging for the beds. I have quite a lot planted underneath apple trees...and self seeded plants seem to be popping up here and there.. ;D