You have put an input duty on Cost+Shipping, and then applied a VAT to (COST+SHIPPING+INPUT DUTY) to get £1354?
The duty and VAT would be paid on the product value not the invoice total I think, and I don't think you pay VAT on the INPUT DUTY charged
So no VAT on shipping (paid to an non-UK shipper), and similarly no INPUT DUTY on shipping charges.
So if product is £675.75, Cost of that would be £675.75 + £130.35 + £130.35 (if your 20% DUTY is right) + 288.35 Shipping = £1200.80.
I think Duty might actually be 10%, but even then cost is £1,135, which I would say is much too close to the real thing to make it worthwhile to take the risk on a copy product with zero backup and warranty.
As Del says, they do a Diesel.
My link was just to point out it is a Truma ripoff, even down to using the Truma Controller example pic with the Truma logo edited out (I bet the actual picture of the "JP Heater" product is actually the Truma as well).