Advice: '87 1.6TD coolant top up

On the way back from France, the red light on my temp gauge began to flicker. After filling up the expansion tank with 5 litres of pink coolant + a little water a few times, I then ran out of coolant and had to fill the expansion tank with water.

Back in the UK, I replaced the blue expansion cap but still the red light came on. My mechanic replaced a faulty thermostat. He bled the system and said there was still coolant in the system.

After a 40 mile drive, the expansion tank & top up level was the same as when I left. The temperture gauge registered just below half. No flickering red light!

Passing a VW garage, I had the coolant level checked for strength and they said it was at it's lowest strength level.

I went back to the mechanic who said I could simply drain some of the coolant by removing the hose from below the alternator.Then simply fill up the expansion tank and top up tank with pure pink coolant, drive it and then fill up again.

Question:

Will that be ok?

Many thanks

Many thanks!