I notice in this chart that the earlier years of the decade (2000-2003) are far more represented than the later years (2006-2008) which is probably down to the deflation of singles sales. By using Excel I generated what I think the sales list would look like if sales had remained constant since 2000:
1 1. Will Young - Evergreen/Anything Is Possible (1,789 million)
2 3. Tony Christie - Is This The Way To Amarillo (1,182 million)
3 7. Shayne Ward - That's My Goal (1,081 million)
4 5. Band Aid 20 - Do They Know It's Christmas (1,138 million)
5 2. Gareth Gates - Unchained Melody (1,338 million)
6 12. Leona Lewis - Bleeding Love (880,000)
7 11. Gnarls Barkley - Crazy (926,000)
8 13. Leona Lewis - A Moment Like This (825,000)
9 4. Shaggy - It Wasn't Me (1,179 million)
10 6. Kylie Minogue - I Can't Get You Out Of My Head (1,084 million)
11 8. Hear'Say - Pure & Simple (1,079 million)
12 9. Bob The Builder -Can We Fix It (1,009 million)
13 36. Rihanna - Umbrella (563,000)
14 10. Atomic Kitten - Whole Again (956,000)
15 15. Enrique Iglesias - Hero (768,000)
16 37. Shakira - Hips Don't Lie (561,000)
17 44. Mika - Grace Kelly (522,000)
18 29. James Blunt - You're Beautiful (592,000)
19 14. DJ Ötzi - Hey Baby (776,000)
20 21. Nelly&Kelly Rowland - Dilemma (706,00)
21 41. Crazy Frog - Axel F 550,000
22 16. Westlife - Uptown Girl (756,000)
23 60. Leon Jackson - When You Believe (472,000)
24 24. Black Eyed Peas - Where Is The Love (628,000)
25 51. Scissor Sisters - I Don't Feel Like Dancin (495,000)
26 34. Eamon - I Don't Want U Back (569,00)
27 18. SClub7 - Don't Stop Moving (731,00)
28 55. Take That - Patience (488,000)
29 69. Mark Ronson - Valerie (445,000)
30 48. Madonna - Hung Up (506,000)
31 23. Elvis vs JXL - A Little Less Conversation (636,00)
32 76. Nickelback - Rockstar (412,000)
33 62. Take That - Rule The World (465,000)
34 35. Gary Jules -Mad World (569,000)
35 57. Pussycat Dolls - Don't Cha (483,000)
36 39. Gareth Gates - Spirit In The Sky (558,000)
37 27. Shakira - Whenever Wherever (606,00)
38 17. Eminem ft Dido - Stan (731,000)
39 28. Girls Aloud - Sound Of The Underground (596,000)
40 79. Timbaland & OneRepublic - Apologize (407,000)
41 31. Las Ketchup - Ketchup Song (583,000)
42 98. Duffy - Mercy (378,000)
43 85. Sugababes - About You Now (401,000)
44 19. All Saints - Pure Shores (709,000)
45 20. Baha Men - Who Let The Dogs Out (707,000)
46 32. Gareth Gates -Anyone Of Us(Stupid Mistake) (575,000)
47 74. Snow Patrol - Chasing Cars (421,000)
48 67. Nizlopi - JCB Song (449,000)
49 47. Will Young - Leave Right Now (510,000)
50 49. R Kelly - Ignition (Remix) (505,000)
51 99. Timbaland - The Way I Are (376,000)
52 30. Shaggy -Angel (589,000)
53 22. Sonique - Feels So Good (645,000)
54 43. Eminem - Without Me (523,000)
55 61. Kelly and Ozzy Osbourne - Changes (471,000)
56 46. Eminem - Lose Yourself (513,000)
57 83. McFly - All About You (403,000)
58 25. S Club 7 - Never Had A Dream Come True (616,000)
59 40. Wheatus - Teenage Dirtbag (552,000)
60 87. Sugababes - Push The Button (395,000)
61 26. Robbie Williams - Rock DJ (612,000)
62 50. Liberty X - Just A Little (501,000)
63 52. Daniel Bedingfield - If You're Not The One (494,000)
64 53. Ronan Keating - If Tomorrow Never Comes (493,000)
65 91. Akon -Lonely (388,000)
66 92. Westlife - You Raise Me Up (387,000)
67 42. Daniel Bedingfield - Gott Get Thru This (529,000)
68 81. Eric Prydz - Call On Me (404,000)
69 100. Daniel Powter - Bad Day (375,000)
70 45. Afroman - Because I Got High (521,000)
71 33. Spiller ft SEB -Groovejet (Why Does It Feel So Good) (573,000)
72 38. Leann Rimes - Can't Fight The Moonlight (560,000)
73 68. DJ Sammy - Heaven (446,000)
74 59. DJ Pied Piper & The M.O.C. - Do You Really Like It? (476,000)
75 73. Nickelback - How You Remind Me (423,000)
76 63. Moulin Rouge - Lady Marmalade (464,000)
77 78. Atomic Kitten - The Tide Is High (409,000)
78 80. Holly Valance - Kiss Kiss (405,000)
79 82. Scooter - The Logical Song (404,000)
80 54. Craig David - Fill Me In (490,000)
81 56. S Club 7 - Reach (483,000 - 488,000)
82 58. Gorillaz - Clint Eastwood (477,000)
83 72. Geri Halliwell - It's Raining Men (424,000)
84 97. Cheeky Girls - Cheeky Song (Touch my bum)(379,000)
85 64. Gabrielle - Rise (458,000)
86 65. Eminem - The Real Slim Shady (456,000)
87 66. Modjo - Lady (455,000)
88 70. Britney Spears - Oops!...I Did It Again (439,000)
89 71. Destiny's Child - Independent Women (437,000)
90 93. Atomic Kitten - Eternal Flame (385,000)
91 94 .S Club 7 - Have You Ever (382,000)
92 96. Bob The Builder - Mambo No.5 - (380,000)
93 75. Craig David - 7 Days (419,000)
94 77. Bloodhound Gang - Bad Touch (410,000)
95 84. Melanie C ft. Lisa Lopes - Never Be The Same Again (402,000)
96 86. Madonna - Music (399,000)
97 88. True Steppers ft. Dane Bowers & Victoria Beckham - Out Of Your Mind (394,000)
98 89. Ronan Keating - Life Is A Rollercoaster (391,000)
99 90. Westlife - What Makes A Man (390,000)
100 95. Madonna - American Pie (381,000)
Basically by taking year 2000 as '1' I then multiplied sales in 2001 by 1.11, in 2002 by 1.22, in 2003 by 1.33 and so on up to 2008 by 1.88. So if Duffy had released 'Mercy' in 2000, all things being equal her sales would have been around 711,260 rather than the 378,000 she has sold in 2008.
I think this chart is more representative of the sales popularity of songs in the last 9 years. Any opinions?