Bike screw

[Toby]

Help!

I'm trying to find a screw to fit a pushbike but I've not got a clue what type of screw is required and I'm wondering if it's a standard size across all bikes which someone here might know.

It's the one used to secure the front of a pannier to the frame just below the saddle. There's a kind of "tube" welded to the frame on each size inside which is a screw thread but for what exactly?

Anyone got any ideas or am I destined to work my way through B&Q's screw department hoping to get lucky?