Mine did this a few months ago, see my issues..had to pull it back up.
It's not the DMF. It might be the release bearing..but I suspect as I found out yesterday, your hydraulic fluid will be fairly badly contaminated. Dave at Intaservices changed it for me £61 and said that the seals in the master cylinder tend not to be as effective at keeping moisture out of the brake fluid. After that it's still a 'bit loose' at the top end, but everything feels much much better!
I am at 54k, and i suspect that as mind started doing it a few months ago...that it would have also been around the 51k mark!
Replacing the DMF, with a normal flywheel would be a bloody awful idea! The DMF is designed to iron out the extremely lumpy way in which a diesel engine turns the crank, in surges of torque. Replace it with a non DMF, and you'll transmit that horrible constant loading, unloading onto the clutch and the gearbox..it'll drive like dog poo!(and sooner or later the £200 you'll have saved on the flywheel will turn into about £4000 for new flywheel, new clutch, new gearbox)