Thanks, I was looking for a definitive answer! Instead it turns out that getting a frame that fits 700 x 50mm tyres was pointless and I should have missed that step and the next 5 , going straight to a DH MTB.
Seriously though, the new frame is great and I did a load of road riding in Yorkshire last weekend. It was more than capable and handled really well on fast descents so I'm happy. I find most things ride the same though.
Having never ridden bigger than 35mm, I don't know whether the jump between 45 and 50mm will be noticeable/majorly beneficial. Any thoughts?