Posted: 19/07/2015 at 15:28
Plums fruit on old wood, so any pruning will mean losing some of next year's harvest. Now is the best time to renovate an old tree but you need to do it over several years and control the 'water shoots' (lots of vertical growth) which will follow any hard pruning by cutting most of them back next summer. More advice on the RHS site:
Is the tree shaded by another larger tree nearby (that's what it looks like in the photo)? If so, the reason it has grown like this is that it it searching for light and you could consider reducing the crown of the larger tree. If that isn't practical then it might be best to sacrifice it and plant a new plum where it will get full sunlight.