Third Gear Crunching

Currently got a 54 plate 2.0 HDi with 100k miles on, shes crunching/grinding third gear, is there anything to try before I fit a replacement gearbox? Also the engine seems to knock when starting up so maybe an engine mount gone not sure if this would cause the crunching? Also the clutch is heavy but from what ive read it seems that's how a berlingo is in general?

Thanks

Mark