How to strengthen root system of existing tree

[Paul Cleaver]

young cherry plums grow quick with a dense crown - now if you have water logged conditions or stoney ground as mentioned or if the tree has been pot bound when planted and combined with one or more of the formally mentioned, then stability problems can occur.

 

I would reduce the crown by 20% and add a guy rope and pull the tree back to its proper position. after a year slacken off the guy rope a little and see how it goes. if it stabilizes, slacken off a little more the following year then repeat the year after. If none of this works then dig it out, and plant another species

[GTS]

Canopy looks quite dense so have you thought of thinning instead of a crown reduction?

That will allow wind to pass through more easily taking the "sail like" factor away, therefor helping your tree to stand strong.