In need of some advice on a matching a sub

Well, the crossovers should cut off frequencies so that the sub and speakers aren't generating the same signal at the same volume. This should eliminate the tonal differences you are wary of.
It's sort of like your speakers which have crossovers to allow each driver to work at its' optimal range. 
If you go with a high pass filter, then your Kef's would reproduce frequencies above the cutoff, while the subs provide all frequencies below. It also allows your amp to run more efficiently as lower frequency require more power.

I still want the bass from the speakers tho, they have subwoofers in them and they work well at the moment it's just for that extra oomf when an explosion hits really 

Sounds like you're really after a home theater sub instead of a music sub.  If you still want to use your Kef 4S woofers for bass, set them as "LARGE", so that the general 2-channel bass frequencies are still sent to the Kef speakers.  Then just look for a good high-power / long throw / stiff cone subwoofer.  The extra bass effects (such as explosions, etc.) are generally produced on the ".1" track of the movies and they would not generally be sent to your main speakers unless you didn't have a sub.  At this point, it doesn't really matter to try to match the tonal voice of the Kef speakers, since it is all sub 80hz effects anyways.  For general full range sound (music and general move tones), they will still be sent to the Kefs, so no blending would occur with the sub.

yeah I did think that I’ve currently got my rears and fronts set to large on the settings, the majority of my use is movies but I do on occasion listen to music and as I’m looking to spend a big chunk on a sub was wondering if there was anything out there that would do the best of both worlds, I contacted kef and asked about the 209 subwoofer but I just can’t warrant spending £6k on a subwoofer, I’ve also been advised against goofing for that sub as there is another one on the market and you can get 2 for the same price and with those two it produces a better end product than the single kef I never use to have this issue I used to use he model 4s for stereo using a nad 600w per channel power amp and a hitachi pre amp (both old I know my dads old stuff) I bi wired the two fronts and that made a big difference just not enough wondering if I hoocked the nad up to the dennonif that would make the fronts produce more bass then I could bi wire the rears instead