Thanks for the replies.
Update. The car went into the garage, new belt fitted, battery checked, OK charging @ 14+ volts, switched A/C on and charge dropped to 12.1 and power steering failed. Switched A/C off and power steering returns.
Put new alternator on, and no difference the fault is still there, so something is putting an excessive load on the alternator somewhere.
Any more Ideas.
I'll mention the RPM sensor when I get back to the garage tomorrow, the tensioner was all OK