I have resolved my fan issue. A surface mount component had somehow come off the logic board and as a result the (pwm) fan control signal was not controlling the fan. The fan defaults to full speed when it sees no pwm signal. I was able to see the (monitor) signal was working by using smcfancontrol and the slowed the fan with my finger - the rpm changed. As a result the mac was requesting, for example, 2000rpm, but as the fan didn't see the request it was giving approx. 6500rpm, hence the error code. The missing component was a bridging SMD device. By hooking up an oscilloscope I could see a pwm signal on one side of the missing bridge, but no signal at the fan. As soon as the 2 pads were bridged normal service resumed. Not sure how this component has become detached, presumably when I removed the fan connector. Anyway, it is worth checking for 2 solder pads just above the fan connector - if you see no component then you may have the same issue!