I am a time served electrician and qualified gas and heating engineer with 15 years work experience. Having worked in other areas such as contracting and consulting, I have been amazed at the money driven mediocrity of a significant part of the industry including british gas and their subcontractors. The one thing that can be said for certain about british gas is that as long as you keep ringing they will keep coming out. To talk about power flushing is a little misleading without first talking about Bs7593:2006, Treatment of Water in Domestic Heating Systems. This give several methods of cleansing, however, powerfluhing is the most effective and least disruptive and this is from my own personal experience. Having carried out flushes with gravity prior to buying a machine, I have noticed the huge improvement in water quality myself and would now never use a gravity flush if it could be avoided. The point made about pipe work layouts is only of minor relevance considering that the greatest proportion of systems these days are sealed and have no feed or vent and powerflux is commonly used. Powerflux is a new soldering flux which is highly corrosive and MUST be removed/cleaned away after working. The price of your flush is a question of value. A system must be cleaned until it is clean and that can be upto 30 mins per radiator ontop of upto 90 mins each to set up and remove the machine . I always allow one full day for cleaning if I am replacing a boiler, so any engineer who changes a boiler and powerflushes in a day should be asked how they manage to do this properly. Id rather pay anyone £700 for a full day and proper flush than £250 for an hour as the benefits of a good BS7593 treatment will be felt for years. The critical factor after cleaning is inhibiting - adding a chemical dose to the water to stop any adverse reactions like corrosion or bacterial dmage. Once clean and treated, the system is likely only to need a fresh dose of inhibitor every five years or so. The first machine was built for BG in 92 by Keith, the MD of KAMCO (who are very helpful should you wish to call) and this is where I was trained to use my machine. Taking 80 years to rot is rubbish. I have removed a boiler that was not 3 years old with catastrophic failure, meaning so many leaks and faults, due to poor installation and lack of cleaning. I have yet to return to any boiler I have fitted in 6 years with any leakage or corrosion issues.