Yes, for the winter. These can be planted outside here in London BUT (and its a big but) they will eventually (after 5 or 6 years) be tree like. One of my clients put a homegrown avocado in the garden ten years ago - we had to pay a tree surgeon to come and take it down (it was 15 feet high).

Well done! yes pot them on as necessary and keep them indoors over winter. You can stand them outside from June til Sept but remember to harden them off, as with anything going from indoors to outside or they'll get checked. You probably won't get any fruit as they need to be a large tree first, but a good conversation piece.

I should add, we never got any fruit on it at all!

Thanks very much. I don't think I'll be growing them for fruit but what do they look like as a plant ? and do they grow very much in a year ?

Well, they're alright to start with, not unattractive, but as they age and get bigger, it's not a pretty sight, I didn't think. Large, leatheryish leaves with a pale brown underside, gawky grower, needs a lot of room sideways too. Might be best kept as a houseplant! I saw a large one in, of all things, a pitch and putt at Yarmouth in the summer - it was all tropical planting, lovely, with one large avocado, and I only recognised it because I'd seen one before. That wasn't attractive either, but then maybe it's just me - though my sisters agreed, more of a curiosity than an attractive specimen.

This week I. found two I had planted outside, and forgotten about, growing quite nicely. Some years ago I had several in a large pot indoors. I planted the stones about about a month apart so they were at differing heights because I found they were, as Bamboo says, gawky. They looked better in a group.

I tend to plant two or three at different stages in a giant pot. This means that the younger ones as they grow cover up the bare stem of the older, taller ones. They don't really seem to mind sharing a pot.

I did finish up with a monster that in the end had to live outside all year, even though it was still in a pot, because we could no longer get it through the doors safely. It finally succumbed to frost one year.

I don't seem to be able to bring myself to throw away the stones which means I am constantly having to present the plants to friends or I would be drowning in them

They are only any good as a small plant and then only if you pinch out the growing point when they are quite young to make a bushy foliage plant. Like many of the above, I can't bear to kill plants, especially trees, so I've kept ours for six years in a huge pot. It's very ugly, has survived touches of frost, and spends most of the winter in an unheated conservatory. But I've no idea why we keep doing this, other than not wanting to kill it.
If you persevere and get one to 60 feet (which is what they grow to in their natural environment) you may get fruit, but you probably need another one as a pollinator! (lol)
A far better tropical tree for the conservatory is a proper guava. Try to find a fruit in a supermarket and germinate the seed. These little trees have much more attractive foliage with reddish stems, and if you can keep them frost free you can even flower and fruit them after a couple of years. We did in the UK, and are trying again in France.