Thanks Bob,
Yes, of course it's centre tank and not tanks! My mistake.
It was burning fuel way too quickly, I fly a 737 in real life so know what the burn should be. I actually solved it by deleting all the key assignments connected with 'mixture'.
Unfortunately though, I still can't get the PMDG to run anywhere near as well as my Ifly 737 so probably just going to stick with that.
Regards
Jon