Extra phone handset like the picture (Overton RG25)

Hi
Our extension handset has broken and need another please. Apparently any DECT handset from a set will do, this happens to be a Gigaset 540.
So if you have replaced a whole system and haven't yet disposed of, one of your old extensions might do the job. It doesn't need the master base which might have answerphone etc., just the basic handset please.
Thanks for reading