RE: Kombat 107
You've gotten terrible advice here. The K97 was the right call to match the C133 (and you did good). Jumping down to a K87 from a C133 is too big a jump (from ~72 down to ~58 cm width is too big, an 8 cm gap is better).
Now what you need is more fins. Get yourself two aftermarket fins (the SB stock fins are pretty good, but pale in comparison to aftermarket designs). I personally favor True Ames, but don't know if they're available in the UK.
Set up the C133 for light to moderate winds, planing 7.0 and lighter. More important in the fin selection is water conditions, which in this range don't lend themselves to much more than B&F / slalom on a C133. A big pointer, for light air, and a swept fin for ~7.5 are the right calls.
Then set up the K97 with fins for 6.5 and for 5.0 to 5.5, and you'll have two boards to cover a pretty huge wind range (5.0 to 8.0). The windier you get, the more you can steer toward wave designs - the fin with the K97 ought to be pretty good in high wind (if it's anything like my K96 stock fin). It's almost surely too small for 6.5, and you'll be rewarded for getting something a little bigger to suit the lighter wind end of this board's range.
In sum, with two proper sets of fins (a set for each board), you'll be very, very happy with what you've got. With the K87 and C133, I think you'd still have a gap in your quiver in the 6.5 range, no matter what fins you get.