THE invitation has been on the table for five years — it has just been a question of when it is accepted. A month after Frank Lampard played his final game for Chelsea, Jose Mourinho revealed details of a conversation with owner Roman Abramovich. Mourinho, then Blues boss, said in 2014: “Mr Abramovich has left the door completely open for him, on the understanding Frank can come back when he wants in the way he wants.” There was not a man, woman or child in football who did not expect that one day it would end with Frank sitting in the manager’s office. ‘A NEW BOOT-ROOM DYNASTY’ And similar to Steven Gerrard eventually becoming Liverpool boss, it is a footballing marriage which everyone believed was merely when — not if. The biggest certainty since Noah held his hand out and said, ‘It looks like it might rain’.Now, with Maurizio Sarri leaving Stamford Bridge — has there ever been a more successful failure? — that time has come for Lampard. All-time top scorer, triple Player of the Year, Champions League and Europa League-winning captain, three titles, four FA Cups, two League Cups . . . and now manager. Only there are people who believe…