4  Goes on again about motorbike's front brakes failing (2-7)

 9  Robin Goodfellow with nothing on in 12/13 novel (7)

10 Added part of Will's fish and chip bill, essentially (7)

11 Most important to fit up computer (9)

12/13       Comedian liking a simple mistake (5,8)

15 Report looks hard at flight (6)

17 Anger about Pence's head being seen in contact with Trump's backside? A little wave, perhaps (6)

18 Master criminal needs pollsters to replace head of political organisation (8)

21/8 Repels Lester, erratic old actor (5,7)

22 Arm supports with stolen weapon (9)

24 See 23 Down

26 Punish company blocking non-working doctor (7)

27 Take apart dreary books in English (9)

28 Football manager's second agreement (5)

 1  Maintained parent didn't sing (4,3)

 2  Winter Olympic venue's very cold greeting (5)

 3  Irritate the frail old man 12/13 helped to bring down (5,6)

 4  Say Hertz works can't grow outwards (4-1-3)

 5  Sort of cake half from religious text (6)

 6/12        Furniture and computers can improve the appearance of cramped accommodation (3-7,4)

 7  Use this to change the current if I finish in the river after sounding drunk (9)

 8  See 21 Across

12 See 6

14 Makes up cheek, hearing river underground (9)

16 Broken nose, in US, is something not worth talking about (3-5)

17 Agent does badly placed in trust (7)

19 Drivels incessantly and eats - try going off (7)

20 View first of some exercises to perform outside (6)

23/24       Songs of Praise presenter's hoarse cry about decoration (5,7)

25 Way of working with America's Big Bird (3)