Took my cylinder heads off today and found this inside one of the cylinders:

The car ran completely fine with no roughness or smoke or any else.

Any ideas?

Burnt oil. Suspect a slightly leaking valve stem oil seal allowing oil to get into the combustion chamber. Did that cylinder look dirtier than the others? I'd be inclined to just clean it off and leave it unless you intend taking the valves out anyway. You should have got valve stem seals in the head set so you'll have them if you do decide to replace them.

I have all sorts of seals in a bag so I would have the parts to do it.

I've never disassembled valve gear though - does it need special tools? I'd imagine a valve spring compressor at the very least.

I'm not a fan of putting something back together knowing there's something wrong with it :(

It did run great before though...

I think I might be murdered in my sleep if I report back that there's another problem/delay on this job!

Left unchecked it will presumably worsen. Worst case the cylinder fills with oil that can't be compressed and goes boom or is it not that bad?

Oh, it only seemed to be in that one.

It'll never get that bad, or at least not before you notice the oil level dropping at an alarming rate and the view from your mirrors obscured by a huge cloud of blue smoke. You'll notice you've got a bag of 16 rectangular section O rings in the head set. They are the token attempt at valve stem seals used on the same engine when fitted to a Classic so they aren't that important.

Yes, to remove the valves you'll need a valve spring compressor (much like the one in my garage had we known) but if you think that is the total amount of carbon build up in the however many miles the engine has done since new, just clean the piston crown and put it back together. You may decide to take that head off again in another 100,000 miles or so, so you can do the valve stem seals then. Mine were done for the first time when the engine came out to go to V8 Developments at 287,500 miles.....