Inspiron 1300 screen black mark appearing

Thanks for the advice, considering the cost of the parts not sure its worth repairing, not good for a laptop less than 3 years old.

By the way the black mark is not there at switch on, it starts to appear after a few minutes, also if I reduce the brightness to minimum it does not 'grow' so quickly

It's possible it may be just the inverter - you'll have to try a Dell parts seller or EBay for one, though, as they're screen specific and not generally sold new without the purchase of a screen.

Alternatively, an LCD repair shop should be able to rebuild or replace a faulty inverter for $100 or so.

Stop using the system until you get it repaired - the heat will eventually permanently destroy that part of the screen if you continue to use it.

Sounds like a failing inverter that has overheated and damaged the screen - you'll likely need to replace both the screen and the inverter.