Hi Steve,
Yes, I took them off, but there was hardly any "lint" on any of them, or in the vents themselves (as far as I know, the car has not really had many rear seat passengers in its life). Funnily enough, I went out in the car last night when it was 15 degrees outside, and the temp went up to 44 after about 10km (showing red on Hybrid Assistant). After we came out from the restaurant and drove for a couple of km towards home, fan still on 3, the temp went down to 39 quite quickly and then up to 40 (yellow on HA) - where it stayed.
According to HA, the batteries seem to retain heat for quite a while when the car is turned off. They are quite dense, I suppose.
I've got the manual for the car, so the next stage was going to be having a look at the fans, but as you say, if something was wrong in that area, there would certainly be a warning light flashing somewhere.
Could I ask where you read that 40-60 is the normal operating temperature for the batteries?
It could be that HA is "calibrated" a bit wrong for the car, and 44 degrees is actually a normal state and should be shown as green rather than red.