I contacted eBay about this and the customer service rep I spoke to advised they were aware of it as an ongoing technical issue that they were working on fixing. They gave me this alternative link to do it, which is a totally different link to the one which is broken. The new worked for me and they said just to keep the link handy if I needed it again in future. 

Hope this helps you guys out!

Retract a bid | eBay

THANKYOU!!

I was having no luck in retracting a bid I placed but your link worked so thankyou.

Just want to add that Yes people need to double check the amount they have entered before placing bids but as I just found out..unintentionally placing a bid for the wrong amount is as easy as a quick tap of your thumb! I placed a bid of $5- and was advised to bit an amount higher than $5.50 so I changed my bid to $6.50. The next message on my screen was confirmed I was the highest bidder with a bid of $56.50. Obviously I had failed to delete the '5' before entering 6.50.. My heart dropped and continued to drop everytime retracting the bid failed. I will in future double, triple check the amount before proceeding.. Lesson learnt!!