The diesel fuel filter often has a small heater which comes in at below-freezing temperatures, and this can be identified by it's wiring plug. Also there should be a small tap at the bottom to allow draining any water out of the housing - remembering that water will sink below the diesel fuel. You could try to see if that tap or bleed-valve has come loose.

A more complex but efficient way to pre-heat the fuel. I've learnt something. Thank you for that.

I got criticised by the MOT chap a few years ago.. They blew the seat out of one of the injectors whilst doing the emission test. At that time Morrisons were doing a '30% biodiesel' which I had been using. The car ran wonderfully smoothly on it. The garage blamed that on the seat failure and the thick carbonisation that they found in the inlet manifold whilst replacing the injector seals and accused me of running the car on chip fat! My own thoughts were that it was simply the leaking injector which had caused the carbonisation, but I would be wary of running vegetable oil now as my pension is stretched too far by car repairs. Doing the mod to permanently pre-heat diesel fuel does not sound good. Can this thermostatic by-pass not be changed on its own? If it comes complete with the filter, and if you're going to have to remove the filter to repair the leak, 'appen it's better to bite the bullet and fit a new filter? After all, diesel filters should be changed fairly regularly, though my car has done 175k miles with only one filter change!