I've been towing different caravans for the last five years with my 4.4tdv8.
Last month I hitched the van and the car started fine and off we went.
After a few hours we pulled into a lay-by for a break.
When we come to set off again the engine wouldn't start. Ignition lights came on but nothing happened when pressing the start button. No clicks or anything.
I started checking fuses and swapping relays around. It was going to be a 4 hour wait for the RAC.
I don't know what made me do it but I disconnected the caravan 13 pin plug from the car and it started instantly as if there was nothing wrong.
I reconnected the van and everything was fine.
It did exactly the same whilst towing last week.
Can anyone give me any pointers at all as to what is wrong?

Possibly the starter solenoid is on its way out and needs more voltage when hot. L405 D350 SE Carpathian Grey D5 19" rims

It's just had a new starter.
Don't understand why disconnecting the caravan plug lets it start straight away.
There is a bit of corrosion on the plug wires that I'm going to clean up which might be effecting the cars electrics perhaps?
Thanks.

There are two 12v feeds to the caravan, the constant 12v to the caravan battery and the ignition on feed via a relay for the fridge etc. I would try and identify which one is causing the issue, on my 405, the two fuses are in the load space fuse box.

Thanks for the info. I will have a look.
I just don't understand why the first start up at home is okay but subsequent start ups are a problem the last two times we've used the van.

Possibly because on the first start, the trailer wont have been detected until you indicate or brake though, that could be a red herring, I cant think of another reason unless the relay is sticking. Have you checked the connections where the cable comes into the caravan, I have found on mine two connections on the feed to the fridge which have become very hot at some point, burnt connectors could be causing a voltage drop.

Checked Brake light Bulbs?
There's a link, and if they're filament types and not 100% it can affect the startup. L405 2014 FFRR Autobiography
Corris Grey
Previously 2011 D4 SDV6 HSE Santorini

Both interesting things to look at. Thanks.