How do I know which size to get for my B’twin Triban 5? 55mm/52mm/50mm ?

where did you get those size options from?

Anyhow if you have standard type road caliper brake they should have pads with a replaceable insert..

if so 99% it will be shimano pattern. so get these:
http://www.wiggle.co.uk/swissstop-flash-pro-green-high-performance-pads/

expensive, but they last ages, work well in the wet and dry, stop that black dust and are kind to you rims.

On this page I can see 3 or 4 different sizes. They are just standard replaceable inserts..

http://www.chainreactioncycles.com/Categories.aspx?CategoryID=556

Hi Rich, no link above?

oops soz its there now…

ok i see..

either way get the shimano swisstops

bound to fit, they fitted my road bike and my brompton

swissstops for me. I have the dual compound ones