I know this has been done and documented by other people on the web, but I thought I would keep a note of how I got on with the process. It seems people get varying results some good, some bad.
I started by getting together a selection of figures that are at various stages of Yellowing. I'd actually just won a lot of ten figures on ebay, all of which were yellow and dirty.
So the first stage has been to give the figures a deep clean in soapy water using a soft toothbrush to get into the cracks to remove the dirt. After this I used lighter fluid and kitchen towel to rub off any black marks and pen. This produced a pretty good result already as the figures were really filthy.
On the left you can see the figures as I received them, on the right is after the deep clean and as you can see some of them are already looking a lot better.
The next stage involves the chemicals.
The retrobright has been a great help to me and is well worth a read.
I'll will post more about my attempts at De-yellowing figures shortly.