Any confidence Birmingham had gained from their midweek victory over Reading in new manager Lee Bowyer's first game in charge, was dented after just four minutes.

Joao Pedro's powerful shot from the edge of the area was parried by Neil Etheridge straight into the path of Sema, who gleefully tapped home.

Watford should have eased further ahead from an 18th-minute corner. Sema's in-swinger was allowed to drop to the ground at the far post where Watford left-back Adam Masina somehow contrived to send his half-volley into the advertising boards instead of the back of the net.

The home side were indebted to midfielder Nathaniel Chalobah for preserving their lead after 24 minutes.

Birmingham had begun to press forward and when Alen Halilovic swung over a cross that eventually dropped in front of Jeremie Bela, Chalobah threw his body into the path of the Angola international's shot.

The clearer chances kept coming at the other end. Chalobah's midfield partner Dan Gosling played a long one-two with Pedro before heading the return cross just over the bar.

Another intricate Watford move eight minutes before the interval would have been finished by Isaac Success had Harlee Dean not stretched to touch the ball away from the striker.

Moments after Success had dallied when through on goal early in the second half and allowed Etheridge to smother his shot, the home side extended their lead.

Gosling swung a 55th-minute corner which was headed home at the far post by the unmarked Chalobah.

The destination of the points and the diverging paths of the two sides appeared set even then. Watford made a double substitution immediately after the second goal, indicating Munoz's confidence that the victory was secure.

Bowyer, by comparison, made a triple substitution in his desperation to rescue at least a point from the contest.

The most telling message came from high above Vicarage Road 10 minutes later when a plane hired by a group of disillusioned Birmingham supporters trailed a banner which read: 'By order of Blues fans Dong must go' – a pointed message to chief executive Xuandong Ren.

It had no effect on events on the pitch. Moments after Munoz had made another double substitution, the players he brought on combined to make it 3-0.

Jeremy Ngakia dispossessed Kristian Pedersen and fed Gray, who raced forward and slipped his shot beyond Etheridge.