Coolant reservoir dark brown...

[Tarquin]

I read that rust coloured coolant can be due to plain water as opposed to a special fluid being used or that there may be a problem with the head gasket?

In my case the the entire coolant reservoir inside is coated with very dark staining and residue that can be partially removed with a finger, the water is a strong rust colour, but there's no suggestion of oil within the mix.

From the above information can anybody give an off the cuff assessment as to whether the problem is most likely down to the fluid or the gasket?

(MOT due in 2 weeks)

[CitroJim]

Yep, plain water or time-expired antifreeze can cause this. If no coolant is being lost/consumed then all is OK with the head gasket.

Just drain, flush and refill with fresh coolant and all will be good...

Be aware that even after a good flush, the new coolant is likely to turn a rust colour again very rapidly... This is nothing to worry about.

[myglaren]

You might also try a dose of cooling system flushing agent from Forte or Liqui Moly.
Also add Forte Stop Leak and Cooling System Conditioner that has lubricants and corrosion inhibitors.