When you have more than one Bacchi base it is easy.  Take the suspect base off the Bacchi.  Carefully present the machined face that the o-ring should be sealing down onto against the same machined face of a second base (also detached from the Bacchi it belongs to).  If the machines faces on both components are true they will not rock around when presented against each other; they will meet perfectly right around the circumference.

If your suspect base is warped it will not meet perfectly with the new base; you will be able to rock the parts slightly when you present the two machines faces together.

The distortion is very small; no where near enough to be seen by holding the face up to a light source and 'eye-balling' across it, but unless the two machined faces meet perfectly there will be enough distortion that the o-ring will be unable to seal sufficiently well around the entire circumference of the warped face to hold pressure.

For what we think is a reasonable GBP16.15 you can order a new base and your Bacchi will be as good as new again.