The BB had been replaced before I got a hold of this one. It would've originally had a cup and cone BB, but had at some point been replaced with a cartridge BB.
From the plastic cups I'd say it's a Shimano UN52, but can't be sure without removing it so I can look at the shell.
To answer your question though; if I were to replace it with a modern equivalent, the it would be a Shimano UN55 or UN300 with a 122.5mm spindle.
Love the Trekker. I remember drooling over magazine pics of this model. My shaky memory insists it was £399 brand new, and was the best value bike you could get with a full Deore groupset at the time ?