Since some people in the UK probably have forced air heating and cooling systems in their houses, there are likely a number of good air conditioning filters available. The link below is to the type I use in my air conditioning system. Select the link for "Best 1" Air Filters."http://www.filtrete.com/wps/portal/3M/en_US/FiltreteUS/Filtrete/Products/Catalog-Air/
These filters work a treat in my A/C system. They claim to have electrostatically charged fibers in them that grab all sorts of microscopic nasty things that would blow right through a normal filter.
They are pleated to increase the surface area available for filtration. If you can manage to work a 1" (25.4mm)thick filter under your intake fans, that would be great. If not, then you can always take some of the material and flatten it out. If possible though, it is best to use the filter while it is still pleated. The added surface area will let it collect more dust and things over a longer time, and you won't be having to worry about changing it so often.
The ones that I use are 20" X 20" by 1" thick, cost about $15 to $18 USD, and last about 3 months in our A/C system. Keep in mind that a central A/C system moves much more air through it than a PC does, probably several orders of magnitude more. And, here in Florida, the A/C will run literally constantly, day and night, at times in the summer. Thus I take the filter's ability to last 3 months under those conditions as an indication that it has a very high capacity before it clogs up.
If you can't find these in the UK, you can always search for a filter that is similar in construction.
PS: Your idea for the external control unit for the PC is pure genius. How were you able to extend all the cables that you needed to extend 5 meters? Did you need any signal boosters, or did just long cables work? And, may we assume that you simply extended the mouse, keyboard, and monitor cables the same way, or was something special needed there.
With eager anticipation I will await your video that explains the making of the external control unit, and the cable (I hope).