That's probably the reason Webasto are discontinuing their 5Kw heater, not hot enough. I was well warned before I bought mine to buy the 9kw heater and I'm glad I did. Diesel is as cheap as chips now, I'm paying £1.30 a litre now and I think it may fall further until the Gov puts the 5p a litre back on.
I thought I had responded to this.
Webasto have discontinued the Thermo Top C 5kw heater and replaced it with an EVO5 which is the latest generation of 5kw heaters. There appears to be a Chinese company still manufacturing and selling the Thermo Top C heater.
These 5kw heaters worked just fine as diesel and petrol engine preheaters in vehicles used in countries with freezing winters.