Hi Alan,
This is a good suggestion. We are of the opinion that keeping the two values close together but not as one value works best for our users, simply because it allows the separation of the two values without the users forgetting that the value is there.
With the re-launch of Canvass, lets see how the votes go for this. If enough people feel its not working for them, we can look into moving it into one.
What's most frustrating about this is that the VAT liability doesn't take account of VAT payments that have been made on the annual flat rate scheme so it's inaccurate.