Oleanders growing wild abroad get baking sun, hardly any water and no frost. Yet they flower prolifically and just look stunning. I think that growing them outside these conditions is going to be very difficult indeed.

This one looks totally pot bound to me, for one thing. Rogi, I grow one of these in a client's garden in West London, its out all year, in the ground, and it came through the last 2 bad winters relatively unscathed, still green in fact. However, it's planted in a southfacing position, directly behind an 18 inch high wall, in a suburban street, so probably quite a good micro climate for it there. Plant these in an open situation where they are exposed to full winter conditions in your country and ours, they suffer enormously.
Turn this one out of its pot and either plant out or pot up into something much deeper and no less wide with fresh compost. You may need to cut it down to get it to put out new shoots from the base too - if you do cut it, wear gloves, the sap can be an irritant and is toxic.

Thanks for your answers. I thought it looked pot bound as well, so when I go down the garden after dinner I will re-pot it.