Thank you to everyone who replied. I've revised the design based on some of the comments made, it now looks like this:
@Cotty, the desk I'm using at the moment has a very similar construction, i.e steel box section of the same dimensions. The top part doesn't have the diagonal supporting struts that you suggested and it has coped for several years with a succession of fairly heavy printers and speakers perched on it. I'll see what the guys at my local yard say, but thanks for the heads up on that.
@Olishea, I've increased the depth marginally from 55cms to 60cms. Again, this matches the dimensions of the desk I'm using at the moment and I find that to be more than enough.
And everyone who commented on the frame, you're absolutely right. After spending some time trawling through pictures of commercially made furniture, I've concluded that the cantilever style construction in the revised design will be plenty strong enough.
Just have to persuade my other half how nice it will look in real life...