I apologise, @TerryGastin. Due to the public holiday today, I'll need to contact Fielders tomorrow morning when they are open.
I look forward to getting this sorted for you as soon as possible. Many thanks for your patience.
Thanks for your help Mitchell
I have just gotten all the parts out and discovered I have 4 lengths of the smaller back channel. The 2 x 2650mm that I previously mentioned and also 2 x 3930mm. Hope this helps
I've organised Fielders to send out to your address the instruction manual for the Freespan Hip Portal Gable (20 degree) from the Centenary range. This will include details on where the back channels sit. I've also requested Fielders verify whether a digital copy of these instructions exists so I can get you an answer sooner.
Thank you very much Mitchell for all your help. I wish they would add more instructions with the kit.