Hi

I have inherited a 2003 XT that had not been used for a while and the battery was dead. I removed it and charged it and car opened but would not start (seemed immobiliser preventing it) but the alarm clearly was OK! After 15 mins of alarms I had to disconnect battery again. Any help appreciated. Thanks

If you have more than 60 seconds delay from unarming alarm to starting the car the immobiliser in the alarm activates (part of cat1 alarm criteria)
what you have do if won't crank when turn key is with  key turned to ignition position (dash warning lights on) press keyfob button again and you probably here a relay click, now twist key to crank and your problem hopefully resolved ...

I'd apply jump leads or jump pack, if the battery has stood a long time then some cells might be too sulphated to take a fullcharge.

Also, following Mr B's suggestions if still no joy and its an auto try it in N as well as P.  On our previous Outback maybe once a year the inhibitor switch would play up in P, moving it to N would see the car start and the issue might not raise its head for another 12 or more months.

I have no idea if you have to depress the clutch pedal to start a manual Foz, but that too might be worth trying, the previous owner could have had such a switch fitted.

If still no joy look around for a secreted switch, i know of cars that have switches installed that prevent the low tension signal reaching the starter solenoid, not fool proof of course but another problem for some low life to have to defeat.

Many thanks will try that today