The surviving episode with Hartnell is "Johnny Come Home", based on the parable of the Prodigal Son. Cliff and his band, the Settlers, squeeze six songs into the 21 minutes of the show. It also features Lynda Marchal, better known now as writer Lynda La Plante, as Johnny's girlfriend at home, and Una Stubbs as his girlfriend in London (desperately doing a regional accent, and with Cindy Kent dubbing her songs, but dancing very well). The Hartnell sections are a decent scene starting at 4:25 and a concluding line at 18:57, with no sign of the health difficulties that made it difficult for him to take on substantial roles after Who.
Considering the symbolism of the original parable, we may reflect that Hartnell's last surviving non-Who appearance has him playing God.
The other surviving episodes are "Up the Creek", an update of the Good Samaritan to include racism which also features a rather weird cover of the Beatles' "Help!" at 9:59; and "Johnny Faces Facts", a rather laboured extension of the mote and beam to a full episode which ends with a song and dance routine in front of a backlit cross, just in case you were wondering where all this was leading.
This not high art, but it's interesting to see what you could get away with in the late 1960s. Well done to Gareth Roberts for uncovering a Youtube video which had actually been online since 2011 - clearly there's not a huge overlap between Whovians and fans of Cliff's more obscure backlist.