Hello, half of Harlow (where I mostly fly) is yellow and classed as "Class D Airspace". I've seen mixed things about this, does this mean I can fly here or not? It just says fly with caution.

If your flight database on your unit is current you should be fine. D I believe is a controlled airspace, so if there are height limit, Dji go app will let you know. the map will overlay the airspace also. I think if you click on the map the symbol on the  far left, looks  like a circle target, will bring up how to display airspace. By default, it is set to show the area and prompt warning.  In my area there is a class C airspace I get the same warning as you and has a flight restriction of 150 meters, I am not sure, but the Airport may set flight restriction also. Just fly safe and you should be fine, you can also call if you want extra peace of mind.

Class D airspace is controlled and you should not be flying in it (without prior FAA authorization).  If you look at a chart at skyvector.com it will show you.  You need to know the floor/ceiling of the airspace, which is noted on the chart.  If it's class B it will show the ceiling/floor to look like a fraction such as 100/40, which is a ceiling of 10,000ft and a floor of 4,000ft , both in MSL (mean sea level).  If it shows 100/SFC, that would mean from the ground (surface) up to 10,000ft.  

I have noticed DJI's restrictions on the areas are much smaller than the FAA's listed controlled airspace.  You can decide your filght options, but you might want to check the charts to see what the actual restrictions are.  Anything that is class B, C, D, or E is controlled.