I think I’ve found the problem.
I returned to have another look at this. As I couldn’t see a split in the spray arm I thought I would test it just in case.
I took it into the garden and connected a garden hose to the inlet and held it at arm’s length. With water flowing through the spray arm I noticed a spray coming out the end where the secondary spray arm nut attaches.
Removing the secondary spray arm revealed this.
I was looking for a split along the arm as I’ve found with other makes, I’ve never come across this.
New spray arm on order to be fitted next week. I’m pretty sure this is the problem but will post an update to close this thread off.