The website on this link shows how to prune a rhododendron. If you read through all the information you will be able to decide what needs to be done. http://www.finegardening.com/how-to/articles/three-ways-prune-rhododendrons.aspx

Remember if the top growth has out grown the container then the roots are likely to be filling the container too. How long has the rhododendron been in this container?

It sounds as if this is a red-flowered variety grafted on to ponticum rootstock, and the root stock is suckering. If left unchecked, you will finish up with a purple rhodo. I would advise cutting out all the stems carrying the purple flowers, and make sure they don't regrow by removing any subsequent shoots that try

Well spotted Andrew. I did not register that Cb was saying this was one plant.

good point indeed Andrew - I had missed that...

Thank you. I can't remember if the red is at the top or the bottom of the plant. I will have to wait until it is flowering and the severely prune it.