Maybe someone can shed some light on this apparently conflicting information.
On pages 79-80 of his book, Andrew Vine writes, "Jimmy Gilbert said: 'It was really due to the success he [Michael Bates] had in Summer Wine that he was offered a job in pantomime at Bristol, and he was having a jousting match. They had these poles and he got struck, and It seemed it triggered off a tumour. He went to the doctor, the hospital, he went to the physio at Bristol City football club to see whether they could do something about it, but it never went away, it got worse and I think it spread then. It was a great shame.' The seriousness of the injury to Bates's groin was not immediately apparent, but it rapidly became clear that he could not cope with the rigours of location work he so loved, being able to walk only with the aid of a stick, and even then with difficulty."
According to
http://www.its-behind-you.com/spotlightbarbarawindsor.html, about a third of the way down the page, Michael Bates played in panto from December 27, 1976 to March 5, 1977 in Bradford. It is certainly the same Michael Bates. There's an image of the poster with his face on it. Unfortunately the poster doesn't show the year.
http://www.its-behind-you.com/bradfordpantos.html shows him performing there in the same season. I don't find any other references to Bates playing in panto. An injury during the '76-77 panto season couldn't have put him out of commission for filming Last of the Summer Wine Series 3, which began airing on October 31, 1976. The latest that series could have been filmed was during the summer of 1976, so the injury would have to have been sustained in the '75-76 panto season to have put him out of commission for the summer of '76.
The web information about the panto season could be wrong and the error perpetuated by the tendency of web sites to feed off one another.
Or '76-77 could have been Bates' second season in panto, with him playing a role that didn't require mobility. (Given the nature of panto, that seems unlikely.) Other web sources say that the tumor resulted in his giving up all roles except the one on
It Ain't Half Hot, Mum. Vine says that he was able to continue in that role because he could play the part seated.
Or since Andrew Vine doesn't say that he verified the information that he got from Jimmy Gilbert, that information might be wrong.