have you checked the voltage going to the starter?

definetly sounds like a faulty starter motor

sounds like a bad starter.check the volts going to the starter,it might
be o,k off the car but when its got to turn over the engine thats when
you need the power.

If you have had the starter checked, then I would suspect a bad earth
connection, or battery connection.
You can check the starter yourself by shorting between the main positive
terminal on the solenoid, to the smaller spade terminal which is normally
powered by the ignition switch. The starter should run when you do this.
Are the lights dimming when you try to start the engine?

Thanks to all who replyed, problem now solved. Thanks to your help

the threded bar that takes the live on the starter was loose on the
iner casing causing weak power supply.

if anyone reading this is thinking of removing a starter from the 16v
xsi may i recommed you you remove the air box, undo the 3 starter
bolts, using your left hand. you can just manage to turn the turn the
starter round so that the terminals are facing you, this will leave
your right had free to remove the 13mm and 8mm nuts without dropping
the starter so have the ratchet and extention bar ready to hand
first.take care not to loose the starter as you could risk damadge to
the rad.once clear of the terminals you will need an assitant who can
reach up from underneath to take it from you. very fiddily job but in
my oppinion the best way. aproximate removal time is 20 mins. on
replacing be sure to offer it back up terminal end first and connect
the terminals first, if not you will be taking it of again