Sorry for not updating this half of the thread - I made a cross post on the Tech forum but forgot this one
It turned out to be a 'no-brainer' in the end as the system is rated at 1.25 litres and dealer price for 1L was 8GBP.
I sucked the old fluid out from the reservoir (measured at around 4/5 1L) and it was black - however filter was clean and no gloop in the bottom.
Replaced it with nice new MB MH-Z fluid and now my levels are back to max - it was quite a way under the min before
Tip: according to MB data, measure the fluid level when top is within the compartment
Lea