I assume and I'm only assuming, that when the rear belts are fitted new that there is no slack or very little, I took off the cover of the front bank (closest to the radiator) to see if I could find out the manufacturing date using the 8 digit code, I could only find a 7 figure code
the belt on the driven side was quite tight however on the opposite side it was slack, a bit like the old 'V' belt that used to drive a dynamo if you know what I mean, I don't think it will skip but it is slacker.
And yes, as I said previously, the belts are being replaced.