Finally got chance to remove the head and inspect.
Comments please on next steps, at least a valve needed as far as I can see. Is the wear enough to be the issue?
I would suspect that the seat is toast as well. There's a lot of erosion in that chamber that could cause hot spots and detonation if not addressed I would be looking for a replacement head, cheaper than getting that one repaired - unless someone knows different.
Mine was like that (more at the spark plug) and had quite a build-up of carbon behind the exhaust valves too, restricting gas flow. I scraped it all off, lapped the valves and scraped a fair amount off the top of the pistons (careful-soft aluminium)
I have been cleaning the head up when I have chance with brillo pads and mr muscle.
A work colleague looked at the photo and suggested having hardened valve inserts so I have contacted a company for a quote to have the head refurbished and inserts fitted.
Currently quoted £25 per insert, fitted.
£90 plus parts to refurbish the head.