The thermostat with hole drilled was donated by Carl from UK, I'm in HK where weather is a lot warmer than in the UK. It worked pretty well.

I replaced the gauge sender as mine was cracked (for unknown reason, maybe too old). I remember my car had temperature readings sits between the 'M' to 'R' under normal conditions so I agree the senders in more recent times are probably not the correct type (as the case maybe), but nothing much I can do as I already brought from reputable suppliers i.e. MSD and Burton.

We did thermos gun test on the thermostat housing and radiators during operation, all seems normal and no signs of overheat.

Seems I just have to accept it haha.