Thanks again for everybody's thoughts on this - it is really helpful to hear what other people's experiences are.
The news is that I have actually bought an overlocker! I went to one of the local dealers and I honestly wasn't expecting to buy one there and then. It was intended to be a fact finding mission, even though I had done quite a bit of research on line and had all these comments on here.
I played briefly with the Brother 2104D which did indeed sound quite noisy but it was a more tinny sort of noise that didn't inspire confidence. I also was shown the Jaguar machine which had previously caught my eye and was recommended by
@Greybird (thanks). The front opens up fully for threading and I had noticed the left hand front section was made of metal. It feels solid and sounds quieter and more professional and produced a better stitch.
So in the end I went for the Jaguar 99 which does have the 2,3 and 4 thread options. I do not know whether I will use the 2 thread feature but at least it will give me that option in the future and it wasn't that much of a premium.
So far I am pleased with it although it will take a bit of time and use to get used to it. Will see how it goes.
Thanks again everybody for your thoughts and I will post an update when I have used it more.