True bud-weis, but this morning I've found another probable cause:

One of the CPS trigger lugs on the crankshaft pulley broke off yesterday - I used one as a brace whilst torquing up the pulley itself and it must have sheared off in the process!

I've found it on the garage floor and I've stuck it back on with epoxy. Just waiting for the epoxy to cure and then I'll see what happens....