Originally Posted by
stut
The slightly quirky alternative is to take the Central Line to Lancaster Gate, walk to Paddington and then take the HEX from there. The Central Line is faster and more reliable than the Circhampolitan, generally speaking.
However, if you're not confident, then changing in Paddington is bound to be easier. The change from the Underground (Over Ground) station to the main station bridge is dead easy.
The Lancaster Gate plus walk is the one I'm most familiar with: some years ago I had a number of commutes to Cardff, involving a walk or bike ride through the park via the Lancaster Gate triangle and on to Paddington at the start of my journey.