So in terms of using the Chase voucher is there a limit to the number of sectors than can be booked using it? I couldn't see anything in the T&Cs that specifically mention this.

Reason for asking is that I am wondering if I could use it to fly DFW - MAD on IB and then MAD - LHR - NCL on BA with the voucher reducing the number of Avios by 50%

DFW - LHR - NCL and return on BA operated flights would be 190,000 Avios plus $1800 if using the voucher. The DFW - MAD - DFW sectors with IB would be 125,000 Avios and $595, with the MAD - LHR - NCL - LHR - MAD sectors being 73,500 plus $8 if booked on all BA operated services. So you'd pay an extra 8500 Avios but save roughly $1200 (assuming you get RFS pricing on the BA operated flights).

I'm also assuming that as this would be BA + IB then it wouldn't be a multi-partner award?