Went to mechanic and he said its very likely to be that the thermostat is stuck and faulty - this makes sense because even when the coolant was boiling and the car was steaming, the temp gauge was still reading just a little below halfway.
The coolant pipes were getting squashed as the car heats up, meaning coolant isn't circulating properly.
I'm away for 6 weeks now starting next week so will get a new thermostat fitted when I get back.
