Patrick Newell was a talented actor who, although limited by his weight, played his characters with distinction and style. In early Town of No Return he was killed off early but provided a solid character. In Someting Nasty in the Nursery he is more prominent and creates a personable character.
Sadly he becomes a victim of writing fatigue in the final season. The overall slapstick nature of the final season did not give an otherwise solid performer the opportunity to develop Mother into a solid role.
While I never viewed Patrick Newell as a headliner or even a lead-performer, he was capable of better than he got in this final season. His other performances are not generally available in the US so my perception of his abilities is strictly limited to his performance in The Avengers.