advent won't boot

My first time on here, so please bear with me...
I've had an Advent 5711 for many years and it's been a good workhorse. My job, (marine engineer) means it's had a lot of knocks and use but until a few days ago, no probs. Turned off one night, next day it powers up, HD light flashes twice, (then stays OFF), all "normal" lights come on, fan runs, then it shuts down after 27 seconds. Then it will repeat this process continuously shutting down each time after exactly 27 seconds.

If I do the same with the battery out, the same occurs, but it goes through the cycle only once.

I've tried all the suggested fixes i can find, ie removing battery, power, holding on switch for a minute, removing memory, HD, blowing out dust etc, and all combinations of the above.
A couple of points though...i'm not sure if the lid switch is good, the battery is NOT good, but this has been the case for a long time, during which time i had no problems.
I thought maybe it was a heat issue, but would have thought the time for it to shut down would have decreased after cycling a few times...?

Due to my location at the moment, i've not been able to check the DC supply from charger, but after being connected for a period the battery light DOES change from charging to full...constant green.
Could all these symptoms be due to a dodgy lid switch, or DC supply, or the 2 combined?

ANY suggestions would be most gratefully received, as I'm sure you appreciate, in my particular job, a very large hammer would normally be the next step...and I do like this laptop!
Many thanks,
Charlie

 

ps
I get nothing at all on the screen during all the above. I connected to another monitor but still nothing.