External Bottom Brackets

The plastic spacing bush that sits between inner bearing surface & crank axle are often made of shit grade plastic. What happens over time is the oil/ grease/ anti-seize that you have pasted your axle in, have infact, been swallowed up by the plastic bush and swell onto the axle.

Older suspension forks used to do the same thing, the very oil thats meant to lubricate & damp them, actually made all the plastic components swell up and seize, jokes eh!

Get a shimano XT or ultegra grade EXT BB or try and find one of the higher end RaceFace or Hope ceramic or stainless bearing BB's. They will last a proper amount of time, though you pay for that, just like anything. Some EXT BB's don't have the plastic spacer, just axle > bearing surface, but obviously, in England, not a great idea.

Also can't stress enough how important facing the BB shell is. Essentially if you don't get it done*, expect trouble, nothing less.
*and done by someone who knows what they are doing

That about sums it up.

I've run a Hope SS BB and Race Face Cadence cranks for over 12 months and 4000 miles. Still smooth and silky like new.