Changing the mileage can be done but only by taking the clocks apart and being a bit brutal with a sharp implement. I remember that there was a requirement to unclip the numbers/mounting shaft, but without a set of clocks in front of me, I honestly can't remember any further details.

With regards the cam belt, if it hasn't been done recently, then it is definately worth doing asap.

You may find it awkward getting the bottom pulley off, but then again you may be OK - depends on if it's got the 3 threaded holes in it or not. There are some people on here who have changed the belt without removing the bottom pulley, but I haven't tried that, so I don't know how easy it is.

Make sure that when you tension the belt that you turn the tensioner in a clockwise direction to tighten the belt. If you turn it anti-clockwise, the belt can foul on the thermostat housing.

Buy a new belt tensioner when you buy a new belt - example price on ebay from Graham Goode (user RS500_2 IIRC) is about £60 posted for belt and tensioner.