Sub-Contract Fee Under Option B

I am working for a groundworks sub-contractor we are working for a main contractor under NEC option B.

We have encountered several delays and the main contractor has requested that our works are carried out on a cost + fee basis.

I have put forward cost + 15%.

However he has stated that in sub-contract data part 2 the sub-contract fee is 0%

So therefore we should be happy with 7.5%

Is this correct could he actually enforce cost + zero fee?