Searching on the web revealed that you could buy a nifty folding viewer with a lens and a mirror that projected the image onto a white sheet for safe viewing. The cardboard version was £45 which I thought was ridiculous so I set out to make my own. I started researching and pretty soon realised that a pin-hole was the ideal lens for this job. It's simple to make and cuts out an awful lot of light!
This is a bit of a free-form instructable as I'm still experimenting and changing things around to get things better. But the bottom line is that I can totally safely and fairly comfortably observe the sun, so my main objective has been met. I think so far I've spent less than £5 on materials, and most of that was in the "I'm sure I've got one somewhere but can't find it right now" category!