[rondeo]

The fluid was at the middle between max and min.

Bought some brass tubing 5/32" (3.97mm) which fits perfectly through the slot and sits on the bottom of the reservoir.
Old car antennas are a good source of stainless tubing too.

Best empty the old fluid from the reservoir first, otherwise it just mixes with the new in ever decreasing amounts.

It took 1 litre new fluid to do the job.

When peeping under the bonnet now and then I shine a torch at the reservoir from above and rock the car a bit to see the level sloshing. Otherwise its very hard to tell.