Hi Adam, that Terraclean might be a good idea, low usage isn't great for these engines, I do less than 10 miles a day now in mine and that gives me about 20-22mpg, I am going on a 'long-run' on Sunday to Gatwick, I will see what it does on that run and report back.
With the low mileage the DPF warning light comes on and I have to take it for a 'burn-up' to clear it, but with over 180,000 on the clock it does well :-)