Bicycle Mechanics - Headset or stem bolt for 18 inch wheel BMX

My mischievous niece has managed to detach the handlebars on his bmx from the frame and has lost the allen bolt that holds it together. I'm not talking about the 4 bolts on the stem/headset(?) that holds the handlebar, but the single allen bolt that screws through into the frame to attach the handlebars.

I'm not sure if this is the right item:

http://www.halfords.com/webapp/wcs/stores/servlet/product_storeId_10001_catalogId_10151_productId_233539_langId_-1_categoryId_210157#BVRRWidgetID

If anyone can point me to a link on ebay UK for the correct item that would be great. I don't know what size the stem is but its a boys bmx with 18" wheels so I presume its a standard size.
Thanks.

That's not what you want. That is for a threadless headset and I assume you have a quill stem. I would assume they are fairly standard threading but it would be best to pop the wedge out and take it to a bike shop to make sure the threading is the same. Or, if you have a good hardware store nearby, see if they sell the appropriate bolt.

Here is a cutaway pic so you can see what I'm talking about:

http://farm7.static.flickr.com/6169/6196423972_7711a3f124.jpg

My mischievous niece has managed to detach the handlebars on his bmx from the frames.
Interesting family you must have. :)

Thanks CACycling, its the Quill expander bolt in the diagram. This is a picture I found:

http://www.mailorderbikes.com/m5b591s223p8815/RALEIGH_Standard_handlebar_stem_bolt_wedge

Oddly, I can't find any on the ebay UK site. I'll go to a hardware/bike store and ask if they've got one.

HillRider, he was trying to raise the handlebars above the max safety height allowed which I have warned him about before, but he went ahead anyway and ended up losing the bolt. He's just turned 9, is v overweight, and likes to ride his bikes (too) fast!

HillRider, he was trying to raise the handlebars above the max safety height allowed which I have warned him about before, but he went ahead anyway and ended up losing the bolt. He's just turned 9, is v overweight, and likes to ride his bikes (too) fast!
My point was you first referred to a "niece". Usually, that's a girl. Then you said "his". Usually that refers to a boy. :)

HillRider, he was trying to raise the handlebars above the max safety height allowed which I have warned him about before, but he went ahead anyway and ended up losing the bolt. He's just turned 9, is v overweight, and likes to ride his bikes (too) fast!

Would he benefit from a larger bike? He is nine years and trying to raise the bar. Is he perhaps trying to make a too-small bike fit him better?

My point was you first referred to a "niece". Usually, that's a girl. Then you said "his". Usually that refers to a boy. :)

Oh, i didn't know that!

"trying to make a too-small bike fit him better?"

Yes he was. But he also has a correct size mountain bike - the bmx is his second/spare bike.