I would want to know what the fuel pressure was at the carb. Make up a T fitting to go in the line and try to get a reading when the engine is running at various rpm.
HIF carbs do not like excess pressure, keeping them fed with under 3psi at all times keeps them happy.
If someone has fitted an electric pump in place of the stock mechanical one without fitting a regulator, then it will flood all the time.
This car does not have an electric pump – it has the original, glass-domed mechanical pump. it also has the original pump to carb pipe – which has screw fixings at each end and is fairly stiff.
So, to make up a t-piece I probably would have to source some fittings to fix on to the pump/carb with a standard length of fuel hose, etc.
While the idea that it was the pump producing too much pressure did occur to me, I think this is not the case, because when the carb is leaking, I can use the primer on the bottom of the fuel pump to continue that leakage when the engine is not running.
Just an additional note – as I previously posted, this car has been laid-up for 25 years, and prior to that it was all working as expected. So, the only thing that has changed in that time is that it would previously have been running on leaded (or lead replacement) fuel, and now it's running on unleaded E10.
I have serviced the carb with the SU service kit, so all new seals/gaskets - shouldn't be problems there. There was also no dirt or grit in the fuel bowl, so not obvious why it started overflowing, and continues to do so. Oerall, any impediment caused by 25 years of standing idle should have been cured.