Many thanks for the super quick reply. Have just found the plug, run the test and got the following fault codes back:
17 speed sensor (sounds right because the speedo stopped working when the CEL came on)
41 primary oxygen sensor heater
65 secondary oxygen sensor heater wire (black wire)
92 EVAP Solenoid/Valve/Vacuum Lines
I'm a bit puzzled about the link to all these different faults when it was just the speedo not working. The alternator sensor fuse also keeps blowing so I don't know how to start fixing this lot
Thanks again
Chris