After owning the 620ti for a few days, I've noticed the heaters aren't always as hot as they could be, and the temperature gauge doesn't always reach the 'normal' level (just under half-way).
I've had similar on my old Rover 75 CDTi - the thermostats fitted to all CDTi models are prone to opening early with age (you can fix this with an inline thermostat, available here: 75/ZT CDTi Thermostat Kit). The only fix is replacement, however on the 75/ZT CDTi models this isn't easy. Luckily on the 620ti it's a very simple job - the thermostat is located in the top radiator hose on A/C models.
To remove some care is needed - the thermostats tend to bond to the hose, so need careful removal to avoid damage to the hose. I used the blunt end of a normal kitchen knife and a small amount of dishwashing liquid to lubricate it - this allowed me to gently prise the hose off the thermostat.
Here's the thermostat once removed:
It's clearly seen better days, and is slightly open even when cold and removed from the car.
With the new item fitted, the heaters are blisteringly hot (great for clearing the windscreen and keeping your feet warm!) and the engine reaches running temperature much more quickly.
Replacements are available below - they also fit most Rover 420 models fitted with the T Series engine.
Click here: T Series Thermostat - PEL10017