If you have a car with aircon you will find there is less room to do this job. Check Edward McAllister's Easy Alternator Removal guide for another method.

The alternator is situated underneath the power steering pump and is driven by the same rubber belt. I removed the electrical plug. There is also a small cable with an eyelet terminal attached to a stud marked "W" which is the signal wire to the rev-counter or tachometer, to remove as well.  

According to the Rover manual this is how to remove the drivebelt.

Or a long pinch bar, the end goes just under the pulley bracket the heel just rests nicely on a bolt head in the power steering pump bracket.

There are three bolts to undo. One is really difficult to get at the bottom on the pulley side,

One at the bottom on the side nearest the dipstick, (the thing that tells you the oil level, not the mug doing the job!). The third bolt is at the top just underneath the power steering pump pulley. These bolts were very tight on my car. To make things worse was they only have 10mm heads. I split a ½" drive 10mm socket trying to undo these. (When I put the new alternator on I replaced them with bolts, which were the same size and thread diameter but having 13mm heads).

Once you've got the thing undone the fun starts! The alternator cannot be removed upwards without,  (I am guessing) removing the steering pump and its bracket. I didn't like that idea - a lot of work!

You might think it would drop through the space between the engine block and a cross - member and come out below? Well not on my car it won't because the space is about ¼" smaller than the diameter of the alternator.

I solved that problem buy cutting a piece of wood just the right length to move the engine block the required amount on its mountings when it was tapped gently (!) into the gap.

The replacement alternator didn't come with a pulley fitted so I had to use the one off the old unit. You can remove the nut by putting an old drive belt around the pulley and than pinching it in a bench vice. This prevents it all turning while the nut is undone. I had no vice at the time so I had to improvise a bit!

Then, as they say in workshop manual language, refitting was the reverse of the dismantling procedure.

Both the pages in the link below are under construction.
A non-technical explanation (as much as possible!): how an alternator works
Strip down and repair

 Edward McAllister's  Easy Alternator Removal

Being a systems administrator and not mechanically minded, I tend to look for the path of least resistance. Hence, my guide for the lazy:

Tools required:

1. Belt tension removal tool or two very large jubilee clips
1 x 13mm spanner
1 x 8mm spanner
1 x 10mm spanner
1 13mm socket with two way standard ratchet.
1 x 4” socket extension bar.
1 x 8mm socket
1 x cotton bud
1 felt tip pen.

1. Pull the thin water hose that runs from the filler reservoir to the top right (as you’re standing at the front) of the radiator.
2. Block the small hole with the cotton bud. (Sneaky huh?)
3. Using the 8 and 10mm spanners, remove the bolts holding the steel radiator cover in place. These can be quite rusty and liable to snap if a socket is used.
4. Using the felt tip pen, mark the position of the central bonnet release and remove the nut and bolt holding it in place.
5. Remove the steel radiator cover.
6. Unclip the connector to the cooling fan.
7. Remove the 4 bolts holding the radiator-cooling fan in place.
8. Remove the cooling fan.
9. Unclip and unbolt the connectors to the alternator, this is easy with the cooling fan removed.
10. Tea break.
11. Using the belt spring tension removal tool or the two very large jubilee clips, compress the tensioner and remove the belt. (Or use Bill's big lever method)
12. Using a combination of the 13mm socket and 13mm spanner, remove the three bolts holding the alternator in place.
13. Carefully withdraw it by gently pushing the power steering pipes aside and being careful not to damage the radiator.

Time taken: 35 minutes.

As they then tell you in the Haynes book of fairy stories, refitting is a reversal.