I am at my wits end. My vo2 has been steadily decreasing since I started regularly using zwift this (so around October onwards).
I am balancing my rides, low areocbic, high, anaerobic etc, having recovery and rest days but still it decreases.
Does anyone have any tips on how to mitigate this?
Hire a coach? 
That looks like a Garmin screenshot. If so is your Zwift connected to your Garmin account?
Yes it is connected and all rides are showing
Set up a trial/free account in Intervals.icu , import as much data as they will take for free and see if it correlates. If the same, and VO2 is important to you, follow @Mike_Rowe1 's advice.
Garmin also uses the heart rate to calculate the vo2max. Did you ride IRL before and now indoors? This could explain the change, because the heart rate can increase indoors when it is warm and the humidity is very high. At the same power output, the heart rate then increases, which causes Garmin to lower the vo2max.
When did you last do a 5 minute power test? That’ll serve as a pretty good proxy to help you triangulate.
If your most recent one doesn’t compare unfavourably with previous ones then your VO2 Max almost certainly isn’t actually tanking.
Thanks, this could well be a factor
I do have a fan but it may not be enough to keep core temp cool enough
I do recall reading some time ago that Garmin needs GPS data to calculate vo2max (although this was related to running when I researched it).
Maybe that’s why it’s dropping?
It seems very unlikely that your Running VO2 Max would stay steady while your Cycling VO2 Max plummets. Mine track within 10% of each other.
How are your power numbers over this period?
I’m not sure I fully understand the graph! 
The graph shows that your performance in watts and your endurance have not decreased. You were just a bit lazy when sprinting. So get some crazy all-out sprints out and torture yourself for a few minutes🤮. You’ll then see what Garmin says about your vo2max.
How has your FTP changed over the same period?
Are you recording on your Garmin device?
I don’t think you can record in Zwift and upload to Garmin.
The FirstBeat stuff only runs on the Garmin device, so if you haven’t done any hard cycling efforts on your Garmin, that could explain why it thinks your VO2max is decreasing.
I think that’s how it works anyway. I dual record my Zwift sessions using my watch as well as Zwift.
Unless you’re getting proper measurements, it’s not a real number, just an estimated guess via Garmin by the looks of it. Don’t put so much stake in their guesses. W/kg, W/CdA or TTE would be a much more important and useful IRL.
As a Garmin user myself, my experience of VO2max (cycling) is that Performance Condition during an activity is a good indicator of the direction in which VO2max will move.
In order to even obtain a value for Performance Condition you will need to spend several (consecutive) minutes in at least Z3 heart rate. Then, if PC goes positive and even better if it stays positive, you will start to see VO2max trend upwards. Of course, the response is slow, but keep hitting positive numbers and VO2max will follow.
Equally, keep trending a negative (or no) PC and VO2max will probably trend down. Z1/Z2 efforts will not improve the Garmin numbers.
The easiest way, in my experience, to improve PC and VO2max is to ride a ramp up power profile, with power increasing and HR also rising, but the HR rise lagging behind the power. FTP ramp test or similar is perfect.
FWIW I do keep PC visible as a data field whilst Zwifting and I do monitor it. The correlation between PC and VO2max adjustment is pretty strong.
