Still working ideas out...
First thing I need really is the front end off a decent A frame trailer, complete with a good tow hitch that isn't worn (a proper solid cast one, rather than the crappy pressed steel ones).
I need to have a good look to work out how that can then be attached as well though, because that might affect what size is needed. Ideally I think it would be welded into the exposed ends of the sills, but not looked closely enough to see how that can be done, yet. NB: the trailer will be 'unbraked', so needs to stay below 750kg. (I have retained all the original handbrake mechanism though, so we can use that for parking up/storage etc

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Other than that, the main bits will be to look into options around genny's, fridges and solution for the BBQ itself.
Genny needs to be something with some decent power output and one that won't need playing with every time you want to start it (since it will be partially buried away inside the vehicle).
Ideally the fridge would be able to run off 12v and either 110v or 240v, so it can be cooling on the drive to the show (from the car supply) and turn up ready to go
Size is quite a key on both those though, I need to take some basic dimensions for the space available.
The idea is that the genny is well soundproofed inside its own sort of 'sub container' in the trailer, so it doesn't body lemonade us all off while running. That means it will need some thought on getting exhaust fumes and heat out though... I also want to have the genny running off the cars own fuel tank, so we don't have to faff about running out to top it up all the time.
Some heat shield boarding will also be needed, to line bits around the boot area.
I've wondered about having a separate lockable water proof box mounted somewhere in the front of the trailer, that we can set up with loads of phone charge points inside and wired in.
So we'd need some kind of combined charger supply and then all the connectors (probably limit that to micro usb's and the iphone 4 and iphone 5/6 connectors only, these days?)