if its getting hard to start in winter yes do them , ebay is great place for manuals on cd, you print out the page you need and it doesn't mater if it gets grease all over it. Buy a reputable brand glow plug like bosch etc , ebay is fine there too.
One thing that you will not find written in your work shop manual when removing glow plugs.
be bloody patient and do not force the glow plug undone. it will most likely snap off.
First tap them with a socket on them so it shocks a crack in the carbon deposits that have baked in the thread. then try removal . if its still very tight tap some more . remember there just a hollow jug element they will snap if forced and it will often mean a head removal to fix them . I have not broken one but have seen people skip the tapping method and do it, then just charge the customer the labour to fix there mistake.
A cat has no place outdoors. A bird has no place indoors.