Thanks everyone, definitely not a DIY job for us, still earning our DIY chops as this is our first house, so maybe a few smaller projects before I take a sledgehammer to the wall…
Will start to phone around builders and get some quotes. Estimates here are quite similar to what my uneducated guesses were, but timescales much shorter.
My OH works at an architects and has drawn the plans of what we want up, so will try and get hold of them and share them on here later.
If anyone knows/can suggest any builders around Bristol that would be a bonus!