It won't separate, but it may thicken.
The ONLY fuel heater that would help a TD5 would be one that heated ALL the fuel in the tank. The TD5 fuel pump operates all the time, and sends a LOT of fuel round and round the system, many litres a minute to the engine and back, so it would be tricky to arrange a heater.
I suggest you GLOW it a few times, and do NOT press the throttle until the engine is running cleanly at tick-over speed on its own.
Learn the big diesel starting trick - first use the glow a few times, and this will also get the fuel moving because every time you turn on the key the fuel pump will run for about 30 seconds. Now you need to keep your nerve!
Operate the starter and KEEP IT RUNNING, even after the engine begins to fire up, until you can quite clearly hear that the engine is firing on all cylinders, and doesn't need any more help. The starter motor is designed to be able to do this!
Now in a TD5 it is time to let it sit ticking over for a few minutes, during which time the engine will send a LOT of quite hot fuel back to the tank, the pump and the filter, and your cold fuel-oil problems will be over as the fuel warms up.
With a diesel, if you are sure the engine is in good nick and should start, you should operate the starter until (1) the engine is running cleanly, or (2) the battery goes flat. A TD5 on a cold day and with SVO or a diesel/SVO mix, might easily need a full 30 seconds on the starter to get it going. TD5 will keep the glow plugs red hot long after the glow light goes out, to help you get her lit up. KEEP IT TURNING TILL IT RUNS!
CharlesY