Do you mean a flowering cherry (as opposed to a fruit producing one)? Do you know what variety it is?

Just wondering what you imply by trimming.
If the tree wants to be tall and you want it to be small you really need a smaller variety. Keeping reducing the height of a tree that is meant to be taller usually results in an ugly framework unless its done very professionally.

Heavy cutting back should be done in midsummer but you can do light cutting fairly soon if you wish. The danger with doing it at the wrong time is the possiblilty of infection by the disease Silver Leaf.

You can find detailed advice about pruning cherries on line.
Can't say any more really without a photo.

Thanks for the replies 😀
I have no idea what type of cherry it is has it was already here when we moved in 10 years go. It bears no fruit only a abundance of blossom.
I have been told that when we have tried to prune it after blossoming it has just grown talker because it thinks we are trying to kill it.
I will get a photo to show what it's like😀

Sounds like a flowering cherry. It has grown taller not because it thinks you are trying to kill it but because it knows its meant to be a big tree and is not to be deterred. Some of those flowering cherries do get tall and others get very wide as well. Keeping them small would be difficult without spoiling the shape as they never forget they intend to be big. It sounds like the wrong tree for the purpose. So you have a yearly struggle and (unless you are very expert) an unattractive shape in winter. If you really needed small tree you'd be better replacing it with a smaller variety. Decide how tall you want and then Google flowering cherries,height at maturity, and choose another. There is a very small one called Prunus Kojo no mai which looks like a very large bonsai. Very pale pink single flowers early in the season.Mine has reached about 4 feet in about five years.

You can get fruiting cherries on dwarfing root stock that limits them to about 7 feet. But any tree by a pond results in lots of leaves in the water in Autumn, which you then need to fish out...