Ok thanks for replies.
I've just been looking through some suppliers websites and doing the sums.
It seems to me that if I ordered separate items from a few different suppliers I could source individual components and build a 4040 system with one fibredyne pre-filter for cheaper than I could buy the whole system from a supplier.
Probably saving around £100, although I would have to fit everything together myself.
Has anyone else gone down this route?
Yes I did, but to be honest, I wouldn't do it again.
What you don't count on is the cost of the all the neccessary fittings and hose which quickly add up. Yes, you maybe able to save on an extra prefilter, but on our water supply we still need the sediment filter as a first filter before the carbon block, or Fibredyne in your case. A 20" Fibredyne prefilter will suffice for 115,000 litres of water and our water has a lot of sediment in it.
I originally costed my setup at a £150 saving, but it ended up to be about £50 which wasn't worth the hassle. For example, some prefilters come with the securing bracket, others don't. You then need to source the correct screws. If the screws are too big then you can damage your housing and there's £40 wasted. The brackets didn't come with the filter housings I ordered, so I purchased them elsewhere - the bracket wasn't correct so I wasted £8.00 on that.