Thanks so much Panky - those Bresco clips are for a Humber Hawk but I've now discovered are way too small. Those Rover P5 clips don't seem quite big enough, 14.8mm gap, as my gap measures just over 15mm. I've found some early Vauxhall clips but they've got an offset hole but I assume the Commer clips require a central hole.
I've now got an idea to make up some custom T-bolt clips in stainless but getting the correct hole position seems key to this.