Haha, I was a bit put out by the response to my post from the accountants that seemed to say "Pay us, we'll tell you"
So I was spurred on to find out the answer and reply to myself. As I can't find this written down anywhere else in English I'll put it down here. Here, we go, someone tell me if you think I'm wrong. It's here in Spanish:
https://planificacionfiscalypatrimonial. com/tag/limite-conjunto-irpf-ip/
Cuota*IRPF + CIP ≤ 60% x BaseImponible*IRPF
Basically the fee for Impuesto Patrimonio + income tax can't exceed 60% of the income tax alone, but you always have to pay at least 20% of the Patrimonio charge.
This seems pretty good news for retirees that might be income poor but cash rich.
I'm pretty sure that the graph in the link I provided is the wrong way round - anyone agree?
Anyone have any thoughts on this? Would love to get any corrections or extra detail!