Diagnosing heater issue Gen 3

Hi

I've had my Prius for over a decade now but I've not got a problem I can't diagnose.

There is air coming through the vents however I literally have no heating. If I put it to max temp and full fan speed I get some heat after a while (15mins), even then only the LHS (UK vehicle) passenger vent lets out proper heat. The other vents not so much.

I do not have coolant loss (that I can tell) as it still looks to be where it was weeks ago.