If you are proposing to use a will writer, ask them what level of Professional Indemnity Insurance cover they possess. This insurance is what pays up in the event a professional advisor messes up. Solicitors will always have this, and a reasonable level of cover is �1m.
If they look blank, draw your own conclusion.
If your affairs are very simple, a much cheaper solution is to write your own will. A book such as "Wills and Probate" published by Which? at about �12.99 may be more than adequate for you.