Anybody replaced an hard disk in a wharfdale LP250DTR,

is just as simple as remove the old one and replace with new?

Pretty much, yes . Disconnect the two cables from the drive and connect them back up to the new one. IIRC, you do need to remove the front panel first to get at the HDD mounting screws. Oh, and be sure to unplug the unit from the mains first and do not touch the PSU board.