2005 Vauxhall Combo 13cdti Revving high

Start the van and the revs just rise slowly to 4000rpm put the van in gear dip the clutch and the revs do drop ,a slight press on and off the accelerator again it rises back to around 4000rpm took it out of a drive it happens when coming to a stop again slight tap on accelerator the revs will sometimes drop , putting it in gear slowing the engine down that way goes back to tick over also with this happening it's just lost power