This is a late flowering variety - my book says to cut back hard in early spring, so its a bit late now, but you could cut it back by half or a third.

I copied this from http://www.gardenseeker.com/pruning/pruning_spiraea.htm Spring and early summer flowering varieties include Spiraea arguta, Spiraea Snowmound, S. Goldflame, S. Little Princes, S. Anthony Waterer. (There are others). All of these should be pruned as soon as flowering is finished - See below. All of these varieties are normally finished flowering by end of June (UK). Sizeable 'flowered' shoots are cut right out, It is good practice to also cut back a proportion of the main stems - Prune back to ground level, or just a few centimetres above. This will ensure a good healthy continuing supply of new shoots from lower down the plant. These new shoots will flower in subsequent years. Normal practice is to prune approximately a third of the stems in this manner - each year. It follows that at the end of year three, there will be no 'old' stems left, after this type of pruning Spiraeas.
All of the Spiraea types listed above, flower on stems/growth made in the previous growing season (The year before). If you prune them in the spring ie before flowering - then you will be cutting off all of the flowering stems and flower buds for the current year, and will end up with no flowers!

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Thank you both Bamboo and Scotsgran. I also have two other Spiraea's that i planted and left to their own devices, not really knowing how to care for them. I will now let them finish flowering then try cutting them back by a third as stated. Thank you.

How odd, Scotsgran - my book lists "Little Princess" as a late flowering one. When does yours actually flower, Rohima?

That is why I gave the link just in case Bamboo. You cannot be too careful. It would be a shame to miss out on flowers because of pruning at the wrong time. You do need to be a bit of everything when you are a gardener but most important is looking, seeing and acting. I find this site invaluable because recording what is happening, when, in my garden means I have an aide memoire for the future and because all the good members are posting, we hope when the flowers are current, we can inspect others efforts to compare what we are finding in our own gardens.

It's been hidden behind other plant because when i planted it i didn't know it was going to remain small, so it has been neglected. I have started to sort the garden out again now and noticed it. It has lots of fresh leaves and looking nice and healthy but hasn't flowered yet this year so i am assuming you are correct Bamboo that it is a late flowering species.