I have an pro variety "arctic" freezing kit which works, but if you only want to freeze a few things the DIY kits would be cheaper. They are colder, by the way!
The tapered bungs are all very well but do not always work in my experience. If the tank connector has internal ribs, for holding/turning it, they will stop the conical bung going in. I have been recommended a big ball of Plumbers Mate (aka staysoft putty, etc) will do the job. I can imagine it would be good but haven't tried it. A Speedfit 22mm cap works as a bung on the vent pipe. They come off easily after use.
Don't forget that the air trapped in the vent pipe will "stretch" when water is let out down below, so a couple of pints can come out.
Also don't forget that the idea only works if there's only a single open end!
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