Both of you!
You would normally not need to prune a Stella Cherry, but if it is growing too tall then you can remove the leader to keep it to its proper height. We had to prune ours back a little for a similar reason. I think we did it in Autumn, but memory does not serve me well these days.

Summer is the time to do the pruning suggested by Owdboggy, late summer at that, say August. Late summer pruning inhibits growth. Other pruning should be carried out in late winter - this will encourage more fruiting.

Boss looked in her Diary and says we pruned it on the 1st of September.

thats about the right time depending on the weather as you want them dorement . its either late autumn or very early spring ie before they come out of dormency . theres pros and cons to both but i prefer the latter myself .

Members of the Prunus genus should not be pruned when dormant as they are prone to a fungal disease called silverleaf. My college lecturer always told me to prune in mid to late summer, but RHS suggests April to July as there are fewer spores in the air and the tree is in active growth, meaning it can fight the infection better. I'd go with what the RHS says and prune it about now if you have to.

Thanks to you all

the info I gave in my answer taken from Reader's Digest Encyclopaedia of Garden Plants and Flowers, fruit section.

well i would prune just as thereabout to shoot so they heal strate up basicly thatswhy i dont do at the end of winter like some .