Not including residual heat from cooking, vacuum cleaner, kiddies etc.
I've just bought an Argos mini oil-filled heater for £24.99 (800W)
because I don't like leaving my little computer room (to go and get
food etc) with the fan heater left switched on.

The oil-filled one is heating the room nicely, although more slowly
than with the fan heater. But I reckon the heated oil will maintain
the desired room temperature for longer between times when the
thermostat switches off. It's been on now (on thermostat) for approx
40 minutes and my Tschibo usage meter says it's used 4½ pence worth so
far (11.5 pence per unit here). Room is approx 2.8m x 2.4m.

MM