I haven't been on here for ages as I forgot my password and just read the posts without commenting.
Regarding the fault code when towing, after replacing the injectors, I've not had the problem since and I've been out with the caravan about six times since (ie 12 journeys) over two hours each time.
At the time I found it hard to believe that the injectors would be faulty at 50k miles, but the workshop I used also did a diagnostic and came up with injectors.
I also had the DMF replaced as I was told it was on the way out which you could here on starting as it seemed to spin for a time before firing. It now starts first time immediately.
On the electrics side, before having the injectors done, I took the car back to my tow bar fitter who checked out the circuits (2x7 pin) and found them to be good.
I also now use Redex at every fill and occassionally an ERG and DPF cleaner.
Hope this helps.