any suggestions for a large plant suitable for a south facing conservatory
I use containers that have a water reservoir underneath for plants that like those conditions. I had a beautiful 6 foot umbrella tree, but the vine weevil killed it.
Certified GKH Gardening Expert
For a south facing window in a conservatory, I would suggest taking a look at Citrus trees. South facing should give them more than enough light, the conservatory should stay plenty warm and the water reservoir would keep them watered to their liking.
The benefits would be that they are lovely trees on their own (and there are many varieties bred specifically to stay under certain heights for indoor planting), they get the most lovely smelling flowers and it is possible to grow the fruit indoors with the right light (which it sounds like you have).