First though, we need to know where you want the job doing so that we can match up tradespeople in your area.
Plastering is a skill that needs many years of practice to achieve perfect results, DIY enthusiasts can tackle smaller areas, particularly outdoors or in garages etc where the standard of finish is not too critical, but ceilings and complete interior walls are best left to the professionals. There are two main types of plastering, wet plastering and dry lining. But both methods require finishing with a plaster skim to give a very smooth and durable finish. Plaster skimming is done in two stages, the first stage is the actual application of the plaster as it is spread evenly over the surface then in the second stage (to be done within one hour of applying the plaster) is to trowel the plaster to a smooth finish and then leave to dry ready for decoration. This final skimming stage of the plastering process is the most critical and the most difficult. If a smooth finish isn't achieved, then once the plaster sets it is very difficult to rectify without sanding down or re-plastering the entire surface again. Plaster skimming is best left to professionals with many years of experience to prevent time consuming and costly mistakes, even if you are a very competent DIY enthusiast, it is unlikely you will achieve the same finish as a professional.
Once it is time to contact and hire a plastering contractor, you need to plan well in advance. Good plasterers are extremely busy and will be booked up for weeks if not months in advance, whether you hire a plaster who works independently, or a local plastering company. One of the best ways to find a decent plasterer is to ask around in your local community for personal recommendations from neighbours, family and friends. If that option is not available, use the world wide web, the yellow pages or the local classifieds to search for a plasterer in your area. You should always contact at least three and ask for references along with a full written quotation detailing materials, start date and finish date. Even better, use the free quote service on this site by simply clicking the "post your job" button to get started, you will have quotes from recommended tradesman near you in no time. Once you have written quotations you can compare them and decide which plasterer is the right one for you. Don't choose on price alone, it is often worth paying slightly more for peace of mind, so go with the plasterer offering the best references and examples of previous jobs, that is also offering a competitive, though not necessarily the cheapest, price.
To apply a plaster skim coat to an average room would typically cost from £250 to £400 depending on the size of the room and whether you want the ceilings done as well (note - if you just have walls skimmed with a new coat of plaster the ceilings will look very dated in comparison, best to include ceilings too if possible within your budget). For those plasterers quoting hourly rates rather than a fixed price, expect to pay around £25-£30 per hour.