It was a combination. The roller wheel is worn (has a groove around it). But also the actu
al extruder mechanism was not keeping up the pressure between the wheel and the geared cog - so it was still slipping.

Replacing the whole unit fixed that, but it is handy having spare geared wheels now.

The aluminium mechanism is a much nicer piece of kit, and I will stick with those.

Has however, taken a annoying amount of time to get these issues sorted out + waiting time for delivery of the parts from China. But, hey ho.

Think I just found the heater unit, so I might try one of those.