Every now and then when I turn the key the dash lights up but nothing happens 
Most of the time she works fine, and I can bump start it when it doesn't start.
I have just replaced the ignition switch to no avail.
What can I look at next? Is there any way of testing solenoid/starter?
She is a 75 superbug. Thanks guys.
Before starting the following, ENSURE the car is in neutral with handbrake locked up tight.
So the starter doesn't turn at all? or click?
Have you got a multimeter? There is a little wire going to the solenoid of the starter. If you have a multimeter, check if 12 volts is actually coming
through this little wire when someone turns the key to the 'start' position.
IF you are getting some reading, it could be slightly lower than 12 volts. Knowing that there is 12 volts coming through proves that the problem is at
the starter/solenoid, not in the ignition barrel/wiring in the front.
If you find you are getting voltage at this little wire, but still no response from the starter motor, use a screwdriver to short the big fat 12 volt
battery wire at the starter to the connector the small wire was originally plugged to. If this turns the motor over, consider fitting a relay. It
proves that the starter works, and that the voltage coming through the little wire hasn't got enough OMPHH to activate the solenoid. Often the long
circuit from the battery, to the ignition barrel, back to the starter consumes too much power that by the time the power comes back to the starter,
its not enough to actuate the solenoid.
If when you short the big battery wire terminal to the small wire terminal results in nothing, or just spark (no noise, clicks thumps turning etc),
issue is in the solenoid or starter itself. Consider dismantling, inspecting or replacement.
Another thing to check is the battery terminals are nice and tightly clamped on, as this can also reduce the current to the starter and prevent it
from turning over (my brother learnt that the hard way, after paying for the replacment of the starter motor eek!).
Check the push on terminal on the starter, failing that replace starter.
id check power at the starter personally. i had a wire break above the gear box which caused similar issues.
tested fine with the test light but not with the multi meter.
This has just happened to me after a trip away... very helpful guys thanks