Pot 'em up! Then when they outgrow the first pot, pot them on...and so on.

A local school or the parks department might be interested if you can be patient and get them to sapling size... unless you own your own arboretum? (LOL)

I have a horse chestnut growing in the garden which I grew from a conker quite by accident. I threw them out after Christtmas one year in case anything could eat them but one of them sprouted so I left it where it was until it was big enough to move and it is now about 2' tall. I have now planted it in an old sink and I hope it will have the effect of Bonsaiing (is that a word) it. Good luck with your nuts!!

Yes pot them up, and next year plant them out in your preferred position, i did this twenty three years ago with a few horse chestnut and oak they are very tall now and look fantastic, all the horse chestnuts will sprout if you pot them up try and keep them in cool conditions and acclimatize them gradually, i remember growing these and having leaves by christmas and was concerned they had come up to quick but they survived and are thriving.

Thank you so much Spritzhenry and Patfran...your advice much appreciated!
I have done that 'Bonsaiing' thing too Pat..with a teeny yew shoot I found in one of my pots - I call it my 'Bonsai Yew'!..it's now three years on and doing well :-D
My concern is - would they survive outside if I potted them up and put them outside now?? I know they're both hardy tree species...but these have been in a closed jar on a bookcase shelf in the living room since early November..do I wait a few weeks before I introduce them to the big outdoors??
Thanx again for your answers
cheers
j :-)

I've just typed my reply..and I've just seen your comment Julien...so thank you for your advice too - you must have very green fingers!
cheers
j :-)

Have you got anywhere sheltered for them outside? They should grow on 'hard' outside. They're too warm indoors.

Hi Spritzhenry...I have a 'cold' space in a temporary plastic 'greenhouse' - would this be ok??
(I have sweetpeas I transplanted from where they'd rooted between paving slabs to pots in here at the mo, they've been doing really well for the last couple of months)
ps...No, I do not have an arboretum lol! :-)
cheers, thanx for your advice,
j :-)

Yes, absolutely fine. :-))

Oh dear, and there's me thinking you might have a country estate! LOL.

Lol!! Thanx Spritzhenry for making me smile :-)
Ta for your advice...and have a lovely day
cheers, j :-)

My pleasure. :-))