In reply to alastairmac:
I've got a 500ml Thermos ThermoCafé flask that cost me maybe £5 on sale in Asda and I bought my girlfriend a 500ml Thermos Ultimate flask for ~£25 last year.
Tested them side by side last week. Mine 15hrs after filling and hers 12 hours after filling. Both still full. The water in mine was still hot enough for a cuppa, the water from hers even more so.
I also have a bigger Stanley flask that comes out in winter. Again this keeps stuff hot for 12hrs plus.