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Thanks, Naomi!
You set me thinking and I looked at the nutritional content of the creams and milks. Interestingly, the "Heavy cream" has a greater fat content than whipping cream, whilst Nestles site says that the fat and protein content of evap and condensed are almost the same, so it looks as if the higher sugar content of condensed is the major difference. So it could be down to one of those two and the evidence from your friend if he/she used double cream suggests it is the sugar!