Then try it and compare the result. Before powerflushing became popular, the traditional way was (and still is) standard instruction in all boiler & chemical makers manuals. Attaching a main and opening the other end has never been, at any stage, a method for system cleaning.
PS. A cold powerflush is also as useless as a mains hose one. Besides it's all irrelevant now as the OP has by now gone to that bastion of central heating excellence - the 'Kockney Koi Carp' shop, to get his hose adaptors.
It's the Chemical cleaner, heat plus the force that does the work, not forgetting shutting down the rads and cleaning the circuit and rads one at a time
If you need to find a tradesperson to get your job done, please try our local search below,
or if you are doing it yourself you can find suppliers local to you.
Select the supplier or trade you require, enter your location to begin your search.
Please select a service and enter a location to continue...
Are you a trade or supplier? You can create your listing free at DIYnot Local