Thanks. What are the maths to derive 500 ohms for the base resistor? The relay coil resistance is probably around 360 ohm, giving 33 mA current, so the BC547 transistor will drive that just fine. With an Hfe for that tranny of at least 100, 1mA of base current should drive it well enough. The thing I don't understand is the current output from Q on the 4013 and how its moderation by the base resistor is calculated.
Will now be spending some time ensuring I have the existing wiring to the ECU properly identified, then some shopping around for parts and (the bit I hate) placing the parts on a board (despite the help from the Eagle app, it is somewhat tedious).