Slipping one piece crank

So 1 piece cranks are shite in essence?

I swear it's the BB moving though. I can see it with my eyes!

I haven't put any pictures up of the offending BB but I can do if it will make a difference.

Still don't get how a BB can slip if it's 1 piece and the barings are fine and the cups look decent to me. it wasn't slipping when it was geared but is now, with prev chainring and new one.
Granted the fit on the peg is shite, but it's not this that is moving.

Thanks for your input guys.

No, they're fine for what they're intended for. They aren't intended for fixed wheel bikes. Especially a fixed wheel polo bike.
I don't really get how a bottom bracket can "slip".
It rotates, it's meant to. If it didn't go back and forward it would be stuck?
Only other movement i can envisage from a bb would be play in the bearings which would be a knock or rattle. This would be side to side.