Just done mine, it's a piece of cake when you have done it
1 hour job start to finish if you are good, 2 hours if you are me. I would go a step further and replace the P gasket as well, you will be kicking yourself if that starts to leak after you have done the water pump as you will have to redo the water pump as well... At least then you will have the pump housing completely off so you can clean the surfaces thoroughly.
2-3 hours to do the lot if you know what you are about - 5 for me
Edited to add: that was a 300tdi in a disco, I don't know how much room there is in a 90 WRT the radiator though.
SBS right here:
http://www.landyzone.co.uk/lz/f49/p-gasket-renewal-how-91258.html