Cutting back osteospermums

Last year I bought some osteospermums which flowered all through summer... And winter... And are now growing again! They weren't anything fancy, just from b&q. Im quite happy to keep them going as I like the colour, only problem is that they have kinda flopped forward, and are hanging over the edge of the trough.

Can they be cut back or is this not a good idea, especially as they are flowering?