Hotpoint WD640?

Had this washer dryer for many years but still in good order.
Recently wanted to dry some clothes but after a few minutes I get a F13 error (overheating & thermostat trip).
I have taken the element (coil type) & fan housing off - plastic fan seems ok and not loose, no rubbing marks to suggest any problem, but the housing gets extremely hot and the cut-out trips, then F13 error.
Can see (spindle) and hear that the fan is running and the metal housing is all clean.
I can reset the switch with a paper clip.
Can only think there's a problem after the fan but unsure what to remove/clean here despite downloading a service manual from the internet.
The opening from the drum (behind door) to the dryer housing, elements and fan are all clean & clear.
At the back of the machine, it seems to feed into a plastic housing - this is the bit I don't understand and could be where the fault or blockage is.
Does the fan suck air from the front (drum) or blow hot air into the drum at the front?
Can anyone help please?