It's one of two potential trees, either Salix caprea (Goat Willow / Pussy Willow) or the closely related Salix cinerea (Grey Willow).

The tiny seeds of all Salix have a tuft of hairs and can blow for miles from their parent plant. They must reach damp soil within a few hours of release or they quickly perish. Yours obviously landed in a pot of nice moist compost a year or two back!

It's a Salix caprea (Goat Willow) two years old.
I have two bonsai'd (one of which is in my photos). Keep the compost moist at all times. Feed with half strength multi purpose plant feed every two weeks in the growing season. Do not feed until leaves have been open for 1 month.
Nip out the growing tips to encourage back budding and to shape your tree.
Willows are thirsty trees that grow quickly and are quite forgiving if you trim back too hard.
Congratulations on owning your first bonsai. You've now got the bonsai bug.

Thanks for that prompt response both of you, much appreciated.

I guess if I moved it to a larger container or in the garden it would grow to normal size would it ? Can you clarify back budding for me Stjohntongue ?

My husband has fancied bonsai gardening in the past so I can't wait to tell him!!

Back budding refers to nipping out the growing tips from the ends of a branch encouraging the buds further back along the branch to open and therefore have more branches off that limb which in time will progress into a fine twig ramification.
Let the tree grow to the height that you want before nipping out the top growing tip.
I hope this helps'
StJ.

Thanks for your help, much appreciated. I can see many hours being spent on this website !!!!