18ac. Huge work unit shortly got rid of boss(4) ?M?O. Umbo is a boss in a shield, but I don't get wordplay. 19d. Out-of -date coin found outside church,an Anglican one,accidentally(8). Assuming 'umbo' is correct, the letters I have are B?C?A?CE. Word wizard offers 'By Chance' or 'Bechance', but again I don't see the wordplay. Advice gratefully accepted so I can put it to sleep!