[Craig380]

So the headlight dims, but does the starter motor turn over at all? Do you have to press the start button again to get the starter motor to turn at all?

If that is the case, it's either your starter solenoid is going faulty, or the starter motor itself.

But first, double check that the battery connections are clean + shiny, and also tight. Use a 10mm spanner to tighten the clamp bolts, not a screwdriver.

If that doesn't help, try bridging the two bolts of the starter solenoid with an old screwdriver. It will spark a bit, but if it spins the starter easily, the solenoid is failing. If it still doesn't spin the starter normally, then the starter motor has a problem.