Could anyone identify this white-flowered tree please (planted in the UK), I was told the name and I think it begins with an 'S' but I can't remember it
I'm a bit confused by your description of 'white flowered tree'. This appears to be a very attractive example of Salix integra hakuro-nishiki, commonly known as Flamingo Willow, not to be confused (as it often is when only using the common names) with Flamingo tree, which is Acer negundo 'Flamingo'. It does not have white flowers, but leaves which are streaked with white, pink and green - the extent of this variegation varies between specimens. If it flowers at all, they will be brownish catkins, early in the year.
Your Salix should be clipped over at the top to prevent it becoming top heavy - this is best done in Spring, after it flowers. It may not flower at all though, flowering is not reliable; these shrubs are grown for their leaves. When pruning back, never prune beyond the graft point at the top of the standard stem.
Yup, this is the willow shrub (pink, white and green) called Salix hakuru nishiki...all plants flower, some are un-noticeable like this plant. Grown for the foliage and usually grafted to make a 'tree form'...
Pruning is maybe twice per year minimum. Easy to do. Cut top at height you want to maintain straight across. Using pruning shears, gather bottom and point tips at edge of flattened top, then chop. This should give you a nice upside down salad bowl shape giving all foliage left a chance to produce food for the plant. This willow, similar to Salix purpurea 'nana' will have an open umbrella shape. Make sure the angle to the crown is a very small angle, slightly rounded. Aim for an umbrella shape, not round, not square. The bottom is always wider than the crown. Brush the entire shrub with your hand to get rid of pruned branches and allow branch tips not pruned to pop up. Do this quite often as you prune. Try to achieve a ratio of 1:2. 1 high to 2 wide...for this plant just the canopy.