I no it's a silly question and am sorry if it but you have checked your oil level is correct.
I could be timing chain or may be a few other things
The timing chain tensioners do fail and are prity cheap easy fix it is a known problem.
Item 6 RealOEM.com BMW R53 Cooper S Timing and valve train-timing chain
There are a few good mini tuners dotted around the country depending on where you are. You would be better off talking to them people like 1320 mini have a very good reputation.
But like I say depends where you live 1320 Mini - Mini Parts - Mini Performance Parts
Hope this helps