I’ve just been on the phone to VOSA regarding dual circuit brakes.
It would appear that an article in Total Kit Car has stated that spilt on dual circuit brakes must now be diagonal i.e. O/S front with N/S rear etc. instead of the more traditional front / rear split. The contentious paragraph is 09A section 4 which states,

"The vehicle must be fitted with a split (dual) circuit brake system with each part of the system operating on at least two wheels (one on each side), capable of operating in the event of a failure of the service brake or its power assistance."

I have just had it confirmed that the split can be front / rear or diagonal. Great news as my Riot is front / rear. However I was made aware of a new brake test that has been added to IVA that was not in SVA.

09E section 3
"The secondary brake performance must be at least 30%,(of the CLW or DGW as determined to be the highest) for each half of the split system."

DGW = Design Gross Weight
CLW = Calculated Laden Weight

The VOSA chap indicated that this new test may be difficult to achieve with a front / rear split. The test assesses the effectiveness of the least efficient circuit in the split. With a diagonal split you get one front and one rear brake per circuit – both circuits will have similar efficiencies. With front / rear, it will be the rear axle alone that will have to achieve 30%. OK so you could increase the efficiency of the rear but you still have to pass the dynamic lockup test.

In conclusion, front / rear split is ok but you’ll have to make sure the rear axle makes 30% efficiency without locking up before the front. A possible solution could be to ensure the design weights you supply are as low as possible. More later.