I found that I developed a new problem with the left indicator - It wasn't working properly sometimes and I couldn't work out why. After playing with it a bit I determined that it wasn't working when I am braking. Which was caused by the fact that I pulled out the rear light cluster to check it and didn't put it back in properly because the fuel tank is in the way. Then it wasn't earthing properly so I had a problem. I will fix this up on the weekend.

And now, back to the original problem - I may have worked out what it was that was causing it. There were a whole lot of tiny copper filings on the indicator switch on the steering column from the wearing of the contacts over time. I have brushed them off now, but my guess is that they had built up to the point that they were making a circuit across the switch when it was turned off, causing the indicator to do its random flashing thingy. I will fix the other earthing problem, then test it out on the weekend.