In regards to your bathroom, the cost of a plumber will depend if your changing your bathroom ''like for like'' ie new bathroom going in exactly the same place as your old one, re-directing hot/cold pipes and toilet waste etc pump up the price.
Try to do as much DIY as possible, ie decorating, labouring, taking rubbish to a council skip etc, its all big savings, failing that, get a friend whos either in the building game or has good knowledge of DIY, a couple of beers will surfice if hes a good mate.
Same applys to patio doors, if it ''like for like'' it should be relatively straight forward with a little help with the lifting.
Prices for both mentioned can be almost anything, just shop around, your more likely with �500 to spend on a bathroom best going to wickes or BQ.
Ps, Golden rule for a new house, do one room at a time.