Nice car mate! My mk1 turbo Orion did the same (coolant coming out after shutting). The reason is the hot exhaust, engine and turbo right next to the radiator. If the temperature was high to begin with, then after shut down the turbo will boil the coolant then it comes out through the expansion tank cap and the small hose connected to it. There are several cures to the problem:

Rewire the power to the fan so it continues blowing even after engine has been shut down untill the temperature has dropped.

Build a heat shield between the turbo and the radiator.

Change the expansion tank (or cap only if it fits) to a S2 unit if you haven't already. The pressure relief valve has a higher rating. (I suppose mk1 Orion and S1 have the same cap)

Don't fill coolant to max. Keep it closer to min.

Use a 82C thermostat. Helps ever so slightly. I use a Toyota MR2 unit. It also opens up much more when it opens.


Do you have any pictures of the car? Fast mk1s don't come by every day.