Your DateDif formula has an extra space after the DAY/S part that wasn't being accounted for. Try either:
<b>Excel 2003</b><table cellpadding="2.5px" rules="all" style=";background-color: #FFFFFF;border: 1px solid;border-collapse: collapse; border-color: #A6AAB6"><colgroup><col width="25px" style="background-color: #E0E0F0" /><col /><col /><col /><col /></colgroup><thead><tr style=" background-color: #E0E0F0;text-align: center;color: #161120"><th></th><th>E</th><th>F</th><th>G</th><th>H</th></tr></thead><tbody><tr ><td style="color: #161120;text-align: center;">25</td><td style="text-align: right;;">1/2/2010</td><td style="text-align: right;;">1/2/2010</td><td style=";">0 YEARS 0 MONTHS 0 DAY/S </td><td style=";">£6.00</td></tr></tbody></table><table cellpadding="2.5px" rules="all" style=";background-color: #FFFFFF;border: 1px solid;border-collapse: collapse; border-color: #A6AAB6"><thead><tr style="background-color: #E0E0F0;text-align: center;color: #161120"><th><b>Sheet1</b></th></tr></td></thead></table><br /><br /><table width="85%" cellpadding="2.5px" rules="all" style=";border: 2px solid black;border-collapse:collapse;padding: 0.4em;background-color: #FFFFFF" ><tr><td style="padding:6px" ><b>Worksheet Formulas</b><table cellpadding="2.5px" width="100%" rules="all" style="border: 1px solid;text-align:center;background-color: #FFFFFF;border-collapse: collapse; border-color: #A6AAB6"><thead><tr style=" background-color: #E0E0F0;color: #161120"><th width="10px">Cell</th><th style="text-align:left;padding-left:5px;">Formula</th></tr></thead><tbody><tr><th width="10px" style=" background-color: #E0E0F0;color: #161120">G25</th><td style="text-align:left">=DATEDIF(<font color="Blue">E25,F25,"Y"</font>)&" YEARS "&DATEDIF(<font color="Blue">E25,F25,"ym"</font>)&" MONTHS "&DATEDIF(<font color="Blue">E25,F25,"md"</font>)&" DAY/S "</td></tr><tr><th width="10px" style=" background-color: #E0E0F0;color: #161120">H25</th><td style="text-align:left">=IF(<font color="Blue">G25="0 YEARS 0 MONTHS 0 DAY/S ","£6.00"</font>)</td></tr></tbody></table></td></tr></table><br />
or
<b>Excel 2003</b><table cellpadding="2.5px" rules="all" style=";background-color: #FFFFFF;border: 1px solid;border-collapse: collapse; border-color: #A6AAB6"><colgroup><col width="25px" style="background-color: #E0E0F0" /><col /><col /><col /><col /></colgroup><thead><tr style=" background-color: #E0E0F0;text-align: center;color: #161120"><th></th><th>E</th><th>F</th><th>G</th><th>H</th></tr></thead><tbody><tr ><td style="color: #161120;text-align: center;">25</td><td style="text-align: right;;">1/2/2010</td><td style="text-align: right;;">1/2/2010</td><td style=";">0 YEARS 0 MONTHS 0 DAY/S</td><td style=";">£6.00</td></tr></tbody></table><table cellpadding="2.5px" rules="all" style=";background-color: #FFFFFF;border: 1px solid;border-collapse: collapse; border-color: #A6AAB6"><thead><tr style="background-color: #E0E0F0;text-align: center;color: #161120"><th><b>Sheet1</b></th></tr></td></thead></table><br /><br /><table width="85%" cellpadding="2.5px" rules="all" style=";border: 2px solid black;border-collapse:collapse;padding: 0.4em;background-color: #FFFFFF" ><tr><td style="padding:6px" ><b>Worksheet Formulas</b><table cellpadding="2.5px" width="100%" rules="all" style="border: 1px solid;text-align:center;background-color: #FFFFFF;border-collapse: collapse; border-color: #A6AAB6"><thead><tr style=" background-color: #E0E0F0;color: #161120"><th width="10px">Cell</th><th style="text-align:left;padding-left:5px;">Formula</th></tr></thead><tbody><tr><th width="10px" style=" background-color: #E0E0F0;color: #161120">G25</th><td style="text-align:left">=DATEDIF(<font color="Blue">E25,F25,"Y"</font>)&" YEARS "&DATEDIF(<font color="Blue">E25,F25,"ym"</font>)&" MONTHS "&DATEDIF(<font color="Blue">E25,F25,"md"</font>)&" DAY/S"</td></tr><tr><th width="10px" style=" background-color: #E0E0F0;color: #161120">H25</th><td style="text-align:left">=IF(<font color="Blue">G25="0 YEARS 0 MONTHS 0 DAY/S","£6.00"</font>)</td></tr></tbody></table></td></tr></table><br />