Welcome, Tim, to the OEM forum.
Very simply, before you plugged in the a.c. sensor, the emonPi had no means of knowing the voltage, so (a) it assumed 230 V (yes, I know the UK centre voltage is still 240 V) and then it calculated the “apparent power”, which is always positive. Then, when it had got the voltage, it could determine the direction of the current in relation to the voltage, and so the direction of power flow, and more importantly it could calculate “real power”, which is what you pay for. There’s a full explanation on the first page of the ‘Learn’ section about electricity monitoring: Learn→Electricity Monitoring→AC Power Theory→Introduction→An Introduction to AC Power
There is a way, but there’s not a setting for it. In emonCMS, on the Inputs page, you’ll find the processing for your c.t. input. As the first step, just put a × -1 (under ‘Calibration’) to change the sign of the power.
