Have spoken to Stuart @ Rollcentre who has proven very helpful all the way through this.
He agrees that the front leg should sit on top of the sill and that we can either adapt the "bottom plate" to fit back to the sill with a blanking plate made to fit or have the same kind of fitting as the rear cage.
We are going to go with option 2 so Stuart is going to make and post out a couple of angle brackets for us like the rear ones. Thanks Stuart.
Time is running out though for the car to be ready so the plan now is to get the rear cage in only, get the harnesses in correctly along with the comp seats and obtain an MOT. I must thank Bryce for his excellent work on the car - there's no way I would have the car this far forward this quickly without him.
After the Little Devils we can return to fully fitting the whole cage and then move onto the fire extinguisher system - it's all getting very real now this rally driving busines.