do any of the current designs cater for a heaver crew weight, in particular a heavy helm? 

all up 24 stone 15 of that the helm?

Best bet looking older current is probably a Thin Ice. Canterbury Tales will carry 24 stone, but the design does prefer a lighter helmsman. Of course if you don't mind more bum shuffling a CT will be fine. 

If you are going for a new current boat ie a plastic one, the drop bow Winder Tales (3627 and above) may be better for you as you'll get the bow in the water more easily when sailing upwind.

Have sailed with 13 stone me and 15 stone crew; now sailing with 12 stone me and 7.5 stone crew. No appreciable difference in performance at club or Nationals level, except tendency to nose dive in heavy crew mode. The whole point of the modern Merlins is that they are ADJUSTABLE. Try one - you'll love it whatever you weigh.

NSMII, Steppen wolf, Bad company, smokers satisfaction (moded) have suited me as a 19st helm quite often sailing with 30st in the boat just have a more powerful rig. ie not a chipstow which is designed to loose power for the light weights very early on in gusts even in lightish winds, and don't rake so early.

Dave F