Those ten quid filters are absolutely pants and aint worth carrot,for any install where you are keeping the original pipework and rads i would add a spirotech combi,this eliminates magnetic and non magnetic debris
A powerflush will clear almost all crud out of system and when i do it i use heat as well as a large version of the magnetic filter, i can so the system is cleaned to the best of my ability ,but hand on heart i can never promise to have removed every single bit of debris from the system
Now for ongoing maintenence and the relative large cost of new boiler install it is worth spending an extra 150 quid ,this filter from spirotech will last alot longer then any modern boiler,once installed there is no reason for it ever to break down or go wrong because there are no moving parts inside or anything that can wear out
The vaillant boiler requires a perfectly clean system because of crappy heat exchanger made by ginnoni,it has clearances of 6mm in places and if there is any crud of any kind it will block it very easily and because of the way it works almost impossible to get out,the manufacturers will always blame the poor installer not the poor design of the heat exchanger,the mark 2 of this ginnoni heat exchanger is alot better and looks more like the veissman with alot more clearances ,so if the french can not innovate at least they can immitiate the germans
This ginnoni Heat exchanger is not exclusive to vaillant but used across the industry and the manufacturers know full well the problems with it,i know personally of about 30 vaillant which have had heat exchangers changed out ,one customer has had two and then eventually because he was going mad they changed out the boiler for him