This is starting to make some sense!
Apologies - we have no flights purchased or anything like that. We had just put Jan 1st as a ballpark date around when we could travel but we can certainly travel before.
Ideally what you want to do when filling out the online form is put the earliest possible date you may wish to travel on, and that you know you will be able to travel within 30 days of.
Because if that date has passed before your visa is processed, it will simply be made valid for 30 days from around the time the decision is made.
But by putting a date further in the future, you may be in the situation where you get your visa quickly, but you cannot travel for several weeks, because it isn't valid yet.
If the Visa is valid from day of decision (today) and is dated today, does that mean we have 30 days from today to travel (ignoring intended Jan 1st)? This is possible for us.
Really sorry if this sounds daft - I’m just trying to make sense of it all! 
Yes.
Basically, once you get the visa, the date you originally put on the application form is no longer relevant.
What matters are the 'valid from' and 'valid to' dates printed on the visa vignette in your passport. You can travel on any day between those 2 dates, but not before or after those 2 dates.