Went to load the car with a few things for a short trip, had boot, and both front doors open for perhaps 3-5 minutes.  Got all sorted and then went to start the car.  Nothing.  ALL electrics dead.  Buttons on remote not responding, nothing.  Opened bonnet for about 2 minutes to check battery connections etc, and then closed it.  Everything then suddenly worked as it should.  Very bizarre.  Any ideas what this could be?

Most likely a bad relay.  When you shut the bonnet it could've shaken the relay contacts such that it worked again.

Sounds like you have a loose terminal. Check the battery connections, especially the negative.

it might sound really odd to think this , but this seems like a classic sign of a dead cell in the main engine battery. , the cell will come back to life and then die and then come back to life , when it dies everything is off. and when its back to life everything works , now the big problem is , even when having the battery tested will show its fine , and nothing is the matter. which can be very frustrating. the only way i ever found it to work is to remove the battery plugs which will exposé the acid  in the little holes , inside and then watch for a ripple affect  and that is a dead give away.  that's a good indicator .. by the way i would not recommend you do that its probably just easier to replace the battery then go that  length , i only did that is because the battery on the car i did it on cost a fortune. , but most batteries these days are sealed and can not be opened unless you smash them to bits.

 

IF its the battery do not go for a cheap battery. go for a the correct type , grade , SC , or what ever

 

9 hours ago, unofix said:

Sounds like you have a loose terminal. Check the battery connections, especially the negative.

This is exactly what it turned up to be.  The Positive terminal had been disconnected by the dealer on Thursday, and not properly tightened.  Not good as it happened. 

well at least it was a simple fix

9 hours ago, enginestuff said:

well at least it was a simple fix

Indeed, but the dealership was not so impressed when i told them that they’d left the positive terminal loose.