have just replaced the alternator on my 2.5td 90 as the old one was full of crud and packed up,new one seems fine,but the question is at what point should the needle be on the guage?..at present it sits just above half is this the norm?..ta

it's not that simple. the guage isn't calibrated and if you swapped for another guage it would probably not read exactly the same.
Best way to prove it's ok is get a meter and check that the battery voltage is ok and see what the meter says, then if the reading drops you know you have a problem