New bottom bracket not fitting right

[WeymouthWailer]

I wanted to replace my old Shimano UN71 because when I took it out the cups go a bit chewed in the process and felt that maybe better start afresh with new BB and new cups.
So maybe I'm doing something wrong here, put in drive side first, tightened that down. Put in other cup, screwed it in and it became progressively tighter, thought nothing of that. But with the cup screwed into to touch the frame, the bearings are no longer smooth to turn. Threads are not crossed.
Put back the old BB and even after making everything really tight, spindle runs smooth.
What's up? Is the new BB not as accurate as the old BB? It feels like either the cup or the sealed unit is out of alignment slightly, so when it's tight it's distorting the sealed unit.
Thinking to complain to Wiggle.

 

[accountantpete]

Just a thought - have you got the right width?

BB's can be road 68mm or MTB 72/73mm to match the shell of bike. Measure the BB shell on your bike and make sure it matches that shown on the BB.

 

[WeymouthWailer]

Size is 68mm, it's an old 531 frame from Revell.
It's written on the new BB.

 

[lpretro1]

It might sound daft - but have had similar problem and it was the bb catching on a screw protruding though the shell from the gear cable guide under the bottom bracket. Remove screw - fit bb- then replace the screw (with a shorter one if poss)and it is then ok