I had the same with my car, thermostat was stuck open, replaced the thermostat and that fixed it, the temp gauge was in my case always hovering at like 2/3 on the scale, so it never got really warm unless the car had to work (uphill driving etc..) but it was quite clear from the temp gauge that it was thermostat in my case.

Have used a code reader and I get a 'No codes stored in the module' message...

Think it'll have to be a garage job.

 2004 TD4 Gone
1998 1.8 Gone
1986 LR110 Gone

It's looking at live values for the individual blend door motors rather than fault codes that is of more use. Anyway it appears to have reset itself now so good luck for the future

Not sure which heater system you have in your FL2 but I believe that there is a thermal control module in the system. This has a wax thermostat in it which controls the coolant flow and makes sure that the water supplied to the heater is sufficiently warm.
If it is stuck open, the heater will appear to be cold/cool for a longer time after start up. FL2 XS SD4 Auto 2010 2012-2017 (21k - 91k miles) (MY2011)
FL2 Metropolis SD4 Auto 2014 2017- (16k - 77k+ miles) (MY2015)
Metro in its 11th Year of (Extended) LR Warranty / Full LR Service History
(Expensive, but Trouble/Worry free - hopefully?)