Dodgy Alternater

When I start up my v8 disco' the battery light stays on untill I give it a good rev then it goes out at 3000 revs and stays out for the rest of the day including stop/start. This is taking longer to work each time I use it, I've checked all belts OK and can't spot any loose wires anywhere.

I suppose this is only gonna get worse and end up in complete breakdown soon,I believe a new alternater can cost up to £250 (ouch!)
Does anyone think I'll need to replace it or are there any components on the alternater that can be replaced instead? I'm not very good on electrics so sorry if the remedy is obvious.
Suggestions appreciated, Phil.

Sure its not the battery on the duff? cheaper to try first, get someone elses battery try that first.

If not: LAND ROVER DISCOVERY 1 300 TDI ALTERNATOR 100AMP NEW on eBay, also, Land Rover, Car Parts, Cars, Parts Vehicles (end time 06-Jul-08 17:13:01 BST) less than a ton