How do I set the timing on a Singer 201K?

My 201k has worked well for a long time. Now it knocks hard as soon as it starts to sew although it is perfectly quiet running free. The knock is coming from the needle/bobbin area.
I have fitted a new needle - a German made quality one size 14. I have checked the flat side of needle is correct threaded right to left etc. (I have been using this machine regularly for 19 years so reckon I have got that bit right) it is well cleaned of fluff, thread and the like as well as correctly lubricated, but still she knocks. All I can think of is that the timing has slipped.
Any help on checking the timing would be appreciated.