how long do you let your diesel warm up before you start your journey?

I don't
I read it uses more fuel than is necessary. FFRR TD6 HSE
Adriatic Blue with Parchment leather.

Start up and drive off, keeping the revs low and not using full power until the temperature is up.

I don't but have a 30mph zone for over a mile at the beginning of most journeys so keep the revs low and take it easy. .
Experience is the only genuine knowledge, but as time passes, I have forgotten more than I can remember
Volvo V70 P2 2006 2.4 Petrol 170bhp Estate SE
MG Midget Mk1 1962

Previous: L322 Range Rover TDV8 3.6 2008; L322 Range Rover TD6 3.0 2002; P38A Range Rover V8 1999

People used to leave diesels running for a while before driving but I think the advent of modern engine oils did away with the need.
Diesels should run ok from cold as the glow plugs assist combustion for a while even though the light has gone out.
I start it and drive off.

Like others have said I always drive steady when engine is cold and keep the revs down. I find if you don't it sounds like a tractor and drinks fuel at an alarming rate. FFRR TD6 HSE
Adriatic Blue with Parchment leather.

Slight deviation how often should glowplugs be changed if at all? Present :2008 TDV8 HSE
Gone Audi A5 2.7TDI
Gone Discovery 3 HSE
Gone Mercedes CLK
Gone Range Rover 2.5 DSE

My FFRR has two glow plugs that are faulty, but I'm not going to change them as it starts first turn every time, even in last winters -21c temperatures and I don't wait until they heat up

Apparently leaving it to run 15-20 min helps the turbo, also helps the oil to loosen up.

In the handbook it says to never leave the cold engine to warm up. Creates unnecessary stress on the engine. Same with most manufacturers.

The idle running with cold engine is nowadays considered to be quite bad (not only for the environment) as it takes a lot longer for the engine to warm up. When the engine is cold all clearances are bigger and therefore wear is marginally higher.

The best for any engine these days is to drive calmly, put moderate load on the engine until everything is properly warmed up. Properly warmed up does not mean just the coolant, since that warms up relatively quickly, it also includes the engine oil and that takes a bit longer (typically 15 to 20 mins).

On my last motorbike (air/oil cooled Suzuki GSX 1400) I used an oil temperature gauge and would take it easy until the oil was around 80 C before really opening it up. Mind you a 128 bhp motorbike weighing about 230 kg does not need a lot of opening to be quite fast  MY 2010 5.0 SC Galway green and sand interior!!
Have the Faultmate MSV2 Extreme to be tinkering with the settings etc. !!

Just always reverse in to where ever you park so that when you return to the car you can get in, get ready and drive straight off.

Modern diesels can on most occassions run without the glow plugs needing to fire as their fuel injection pressure is so high and the mist so fine, and the compression so high that its only when the diesel is really gelling that you need to worry about this.

I think there is an old transit engine that doesn't even have glow plugs fitted, i think its the old mid 90's shape non turbo engine(i used to have one and it didnt have a glow plug light and always fired on the first turn).

The myth about warming the oil up for the turbo is tosh, the thicker oil is good for it as i think the turbo uses a sleeve bearing so leaving the engine to idle would generate very little oil pressure for it and even at idle the turbo still spins. The cool down is important. if the engine has been thrashed a bit shortly before you come to your destination then reversing into a car parking space for example, and then letting the suspension lower and the mirrors fold in should be enough for the turbo to spin down and cool down just a smidge.

The engine bays in FFRR's do retain heat quite well to be honest, for the TD6 anyway, though i'd imagine the V8 and SC are even better as the engines dont leave much space in there!!