If they are saving to move into their own home then I would take less/nothing. If they are just living with you and have no plans to get their own place/be self sufficient in the near future then at £1000 a month salary I think £200 ish a month is fair for a parent to charge (it would cost them twice that for the benefits they get 'at home').

We have some neighbours with grown up kids living at home. Apparently they are saving for their own places, but a recently acquired 12 plate Mini convertible and a 13 plate 120D say otherwise in my book. Its none of my business whether they stay or pay any rent but I find it amusing when I hear that they are saving...