Service engineer arrived and removed the ice blockage but also carried out a simple modification that involved relocating the defrost heating element to a lower position that prevents the icing.
The engineer was great and took the trouble to go through the cause and corrective action and gave me confidence the modification was in fact a permanent cure, explains why Hotpoint are still marketing the product but would have made more sense if Hotpoint had been more proactive and contacted customers and carried out the modification prior to the fault being visible.