Lights are on, but will not start, frustrating
 

S500 w140
1994

i came across a starting issue one night at the petrol station, filled up with gasoline, upon starting it sounded slugish (like a flat battery) but started after 2 cranks. Drove to the shop and it failed to start.

All the lights are on, radio, air con, etc. I turn the key and hear One Click.
Tried many times, same. Called the breakdown service, after waiting 30 mins i tried again, and it started. Drove home.

Next morning it started and i went to get the battery charged.

it ran for a week without issue. (it has been a good starter in the past, first or second crank without the accelerator)

Then it happened again, just a Click, this time i had to leave it overnight. Next morning it started, and i drove home where it has sat ever since...

Thinking it was the battery, i took it off and went to charge again, came back and still the same, all the lights, just a click.

So just to rule out the battery i got my friend to jump start me, the lights very bright, but just a click... Not the Battery.

Tried shifting in and out of Neutral, still the same.

Took of the Neutral to the battery and cleaned it.

Played with the key and batteries, doors open and lock fine. i have changed the Key Batteries, just in case.

I believe the next step is the Starter Relay ? How would i know if it is broken or Damaged ?

What would be my next Step ?

i appreciate any help, i wish to pin point the issue before i get the garage involved $$$$$

Yours
Jon