Thanks English Impala, good to know I hadn't been too stupid. I can manage the load of the current car/caravan quite easily as it's to go to a show with friends who can take some of my gear if I feel the weight might be exceeded.
The Mustang doesn't yet have a tow bar and I wouldn't trust the US ones even if they weren't always ugly. I did get as far as getting a quote from Watling Towbars and I think they're the best bet for me if I do ever go that route.
caravan clubs look at 85% of the kirb weight of car for maximum caravan weight loaded. if you've not got your car weight have a look in your area for public weigh bridges, I did my rod at a local steel works with a weigh bridge. best bets to get a tow bar thats got a detachable ball made, don't want to spoil the Mustangs looks or catch your shin, ouch!
An interesting aside on towbars and limits.
Be careful with towbar manufacturers as it can vary the towing limits of the vehicle significantly. For example, I have a Discovery 2 with a factory fitted towbar which has a data plate on it showing a limit of 3500kg which also matches the vehicle data. A chap I know also has a Disco 2 but his has an aftermarket tow bar and that is only plated for 2300kg so, despite what it says on the vehicle data plate, it's really worth checking the tow bar data as well just in case.
The LR factory fitted removable towbar was recalled recently due to more than one of them falling off, some while towing! Corrosion in the socket appears to be the issue, the build-up preventing the removable ball from locking home properly.
The persistent rumour about people being fined for towing an empty trailer that's plated higher than the tow vehicles limit was mentioned in another thread. I replied saying that was no longer the case but couldn't find where I'd saved the document, found it now, too big to upload as a PDF here it is as JPEG, hope it's readable.
I always carry a copy of that letter in the car, just in case "young plod" isn't as aware as the more senior plods!