Hi Muffin and welcome to GoY. You need to wait until after your rhododendron has flowered to prune it or you will lose this years flowers. What until autumn now to move.

Thanks Moon growe! I was aware of this but am unsure if it will regenerate as I have strong sentimental attachment to it.

As long as you don't prune it back too hard it should be fine.

Look for the faint rings where one section of growth ended, and another began. There are usually more dormant buds there, so cutting just above one is more successful than random hacking.