My iPad Pro 12.9 gets very warm whilst charging and hardly charges beyond 42% battery however long I leave it to charge since I updated iOS

I would make an appointment at your local apple store or authorized service provider and get it checked out.

Warmth is normal when charging and using, the benchmark is usually ‘too hot to handle’, if the heat is so much you can't touch it without getting burned then something is definitely wrong.

THat said, it should certainly be charging past 42% so there may be something wrong with the battery.