Hi Marion
I have never flown from Stansted but, I have travelled Great Western Penzance to Liverpool St. via Paddington several times, and it's all very straight forward. The east bound Hammersmith & City or Circle line takes around 20 minutes from Paddington through to Liverpool St. where you would pick up the Stansted Express, it would be just a case of changing from train to tube to train without having to go out of your way. Your train ticket will cover the underground.
Like others have said, travelling up the day before and booking into the Premier Inn at Stansted (other hotels are available) would be your best bet. The flight back, if it remains the same, should land at 13.05 so would give you time to get to Paddington for your journey home the same day.
An overnight stay is an added expense on top of the holiday, either going out or coming back, but for those who don't live near to an airport there's no other choice. I hope you manage to work something out and get your holiday to Calabria.
Avocet