Engine scare - seems to be ok

The gunge in the header tank caused much worry and headache but appears (not 100% convinced yet) to be resolved. We drained the coolant and flushed it through with the garden hose pipe. We thoroughly flushed the block, heater matrix circuit and the radiator. I started asking questions and reading as much as I could about what brown gunge could be a symptom of - I already knew of course, oil in coolant from a blown head gasket. This tallied up with the car getting quite hot on the rolling road so I feared the worst.

Thanks to some excellent guidance from the cobra forum I tried to remain calm and worked methodically.

Checked oil on the dipstick - appeared normal.
Drained oil and inspected it - normal, no emulsified oil (mayonnaise)
Rocker covers are baffled so no mayonnaise there.
Removed all spark plugs. The rear two on the drivers side are a pig to get out as the engine is actually offset slightly so there's barely enough clearance to get the plugs out. For interest, this is what the plugs looked like:

They look a bit white which would suggest its running lean but from what I gather with modern fuels you can't really read spark plugs any more. Either way what it does confirm is that if I'm to make more progress with the carb tuning I desperately need to get my AF meter installed! Anyway, on with the diagnosis:

No coolant poured out of any cylinders (good!)
Plugs were dry, if a little oily.
Compression test - bought a compression tester but stupidly bought one with a very long body so couldn't get it in the aforementioned rear two cylinders so I couldn't check all the cylinders. The cylinders I did check all read 170psi which is absolutely normal.
The important test - coolant sniffer test. Borrowed a kit from a local garage. I had never used one before, you take the cap off the header tank and shove this chemical tester in. If combustion gases are present (i.e. leaking head gasket) the fluid turns yellow. If not it stays blue.
Test fluid stayed blue.

Feeling brave, we put the engine back together, fresh fluids and went for a short 20 mile drive. The hot running problems in traffic still persist but generally everything was fine. Checked coolant again afterwards, no brown sludge.

The next day, feeling very very brave we drove 90 miles each way to Warwick. Gave it plenty of stick, various conditions. Long periods at very light throttle cruise. Some sharp bursts of hard acceleration, some more prolonged stints at higher revs. Got there and back with no issues apart from the expected - hot in traffic.

Following advice, I've now got the header tank filled far less as it was consistently pissing out the overflow but I just kept on topping it up without thinking. the level is now about 1/3 in the header tank. I had been obsessed with getting the header tank as high as possible so that halfway up the tank equates to the highest point in the coolant circuit (thermostat + top hose bend) but of course that's where my air bleed to the header is, and the system is pressurised during operation as the coolant expands, so it doesn't matter. As an engineer I'm ashamed I didn't realise this and needed it pointing out to me.

Anyway, it seems to be quite happy but I can't help be a little suspicious. All of this meant we had to cancel the paint job but we are now back on for painting next weekend.