Service Engine Light

I have a '97 disco with 84,000 miles and the service engine light came on. I have read some of the other threads and have read that it is just a maintenance check up. I called the dealer and they tried to charge me $250 for an oil change, to get the light off, and "other maintenance." I was wondering what the "other maintenance" might be. I have also seen that the service engine light is just for upkeep on the lr and not problems with the engine. So, I was also wondering if I turned the light off myself if it would end up haunting me down the road, or if it is ok?
Any answers help. Thanks.

Check that the fuel filler cap isn't loose, on petrol models it has to be tight, there's a notice on the filler cap.

Peter