That does seem a little low, I have an Ioniq which is notoriously slow, your average charge rate is about 21kW (based on time/kWh delivered).
Even my Ioniq would expect to get higher than that going from 45-85%.in fact I did a very similar charge yesterday and averaged around 28kW.
It may be a lot colder up in Scotland though at the moment.
I don't think you are doing anything wrong, it's either the charger or the charging profile of the car.
What was the charge rate after around 70% charge, some cars do ramp down significantly past around 70/75%, and as you started at around 45%, by the time the battery has been heated up enough to accept a higher rate, the charge then ramps down soon after. It is preferable to arrive at a rapid as low as possible, ideally around 10-20% but that carries other risks such as the charger being broken/occupied!