Right update. Drained the coolant and found a good place to do so of a link On here by removing a pipe on a low point and letting it all come out neatly. Did not bother with any proper drainage points. Then replaced both smaller thermostats. Both were completely knackered and I could not even move them with pliars. The only trouble I had was the clips were the wrong way round so could not get straight on to the spring part. Sorted filled back up and back on the road. Just want a cold star to check it's sorted.