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Pretty sure you have an Italian threaded bottom bracket shell.
It's long been common for Dutch shops to source the bikes they re-brand from Italian builders.
If you had calipers, I think you'd find that your shell measures 70mm x 36mm and the DS has a right-hand (ie "normal") thread, rather than the left-hand thread you find with British/ISO BB.
As the BB you have is British the diameter is less, so the cup falls through the shell.
Why does the NDS engage? Manufacturing tolerances I'd guess: it screws in, but I bet it's pretty loose.
What you need is an Italian threaded BB: instead of "1.37x24T", look for "36x24T". If a BB isn't specifically listed as "Italian", then it's likely "British".
Have a look a Sheldon's cribsheet for more info: the table at the bottom shows you what happens if you mix the wrong shell and BB.
Scilly.Suffolk-
Thanks for that, I never would have thought of it not being British threads.
I guess I'll get it down to a bike shop to get the threads cleaned up anyway, and hopefully they have an italian BB I can try out.
I have a Sugino Messenger crankset that I had planned on using, but I might have an issue with finding a JIS bb with Italian threading? I've never ventured into the world of Italian standards, so don't know if they're compatible with Japanese/British standards at all.
Kgrev
I haven't got a set of callipers, but measured the shell as 69mm wide x 37mm diameter.
The frame is a Davanti, which as far as I know is a generic alloy frame branded by a bike shop here in Holland.
The bb says BB-7420, BC1.37x24T, 68. It's sold as "FSA RPM JIS Square Taper Bottom Bracket BB-7420AL BSA-68-ST"
I've added a photo of the threads as well.