This is a Carradice SQR Slim bag attached to a Brompton telescopic saddle stem.
There are Carradice SQR adaptors for normal saddle bags.
Another view of the SQR Slim on the bike - the bag is behind your legs so presents little air resistance.
This is a home-made bracket to allow a Bike Hod trailer to be quickly attached to a Carradice SQR bracket.
The trailer being towed and when the bike is parked. Since these pictures, the trailer metalwork has had some welding and a bracing bar has been added. I might also shorten the tow bar to get the trailer closer to the bike.
The Brompton is a tremendously British folding bike. They fold down to a very small size (it fits into the boot of a Mazda MX-5!) but can easily be ridden 50-100 miles a day and can keep up with non-folding bicycles with ease.
I use mine to commute to work in Leeds, to go shopping etc., and for the occasional leisure run. I've so far done about 14,000 miles on my Bromptons!