We know this but would Coral ? I have doubts that Coral would really care who did or did not have writing credit as long as all the writers got credit (deserved or otherwise).
Yes, XT came into force on 1st June 1957. So either the record was made prior to June and the M designates an additional levy (M for More perhaps). Or the record was made after 1st June, the engineers at Coral chose to ignore the tax code change and put the M on the label to signify the change in tax. The latter seems unlikely to me, they could have lost their jobs ignoring tax changes. The former sounds odd in that why would Coral make a single then wait 4 months to release it. Unless of course we have our dates wrong and the single was released almost simultaneously with the American release???