Nine times out of ten it's the clutch — usually contamination from a leaking rear main or gearbox seal, or a worn dual-mass flywheel. Less often it's perished engine or gearbox mounts. If the judder only happens when cold it's almost always the clutch plate; if it's there hot and cold, suspect the flywheel.
I get this one in the workshop most weeks. Driver says the car shudders when they let the clutch up at a junction, sometimes bad enough to feel through the seat. It's almost never a "drive it and see" job — clutch judder gets worse, never better.
What's actually happening
When you pull away, the clutch plate is supposed to grip the flywheel smoothly. If anything's contaminated, glazed, or worn unevenly, it grabs in pulses instead of one smooth take-up. That pulsing is the judder you feel.
The usual culprits
Top of the list: oil contamination on the clutch face — almost always from a leaking rear crankshaft seal or gearbox input shaft seal. Second: a worn or failed dual-mass flywheel (very common on diesels with 80k+ miles, especially BMW, VW Group and Ford). Third: collapsed engine or gearbox mounts letting the driveline rock about. Last and rarest: distorted flywheel from a previous bad clutch fit.
How I diagnose it in the workshop
Visual check underneath for oil on the bellhousing first — if it's wet, the clutch is contaminated and a new one alone won't fix it; the seal has to come out too. Then I rock the engine by hand to check the mounts. If both are good and the car's done over 70,000 miles on a diesel, the dual-mass is the prime suspect.
What it'll cost
A clutch and dual-mass flywheel on a typical diesel runs £700–£1,200 fitted in most independent garages. Mounts are usually £80–£250 each fitted. If the seal needs doing while the box is out, that's a few quid in parts — but you'd be daft not to do it at the same time.
Is it safe to keep driving with clutch judder? — It won't strand you tomorrow, but a juddering clutch is on its way out. Once it starts slipping (revs rise but speed doesn't), you've got days, not weeks. Sort it before then or you risk being towed.
Can a clutch judder be fixed without replacing it? — No. Once the clutch face is contaminated or the flywheel's worn, there's no spray or additive that fixes it — anyone telling you otherwise is selling snake oil. It's a gearbox-out job.
Why does it only judder when cold? — Cold-only judder is classic clutch plate glazing or light contamination. The clutch grips harder once it warms up. It's still a clutch job, but you might get a few more months out of it.