I was bidding on Item number:270473968802 and put in a bid of £10. It ended with highest bid of £9.99 so why didn't I win the auction?

You did not win because your bid was too low due to eBay's bid increment system.
Please check this FAQ section for more information:
http://www.myibay.com/login/faq/4/#q14

Sashka wrote:
You did not win because your bid was too low due to eBay's bid increment system.
Please check this FAQ section for more information:
http://www.myibay.com/login/faq/4/#q14

But it said Enter £9.99 or more and I did by bidding £10 and the winner got it for £9.99...

kirkie wrote:
But it said Enter £9.99 or more and I did by bidding £10 and the winner got it for £9.99...

"Enter 9.99" was before someone else bid. Your bid must be higher by at least one bid increment.