Thanks CJsews- the bobbin is in a case and I am able to insert and remove it fine. That's not the problem. It's that with or without the bobbin case inserted, the spool thread is getting wrapped around the hook/shuttle mechanism. The more I look at it, the more I fear that it's the hook needle timing...
The hook should be passing by the needle at it's lowest point, but when the needle is at it's lowest point, the hook is a good inch away still!! :-( So I really need to find a vintage repair manual and see how to adjust the timing... Unless someone can point me in the right direction...? :-)