Hi Colin - if you can manage it, there's quite a lot of information online about the use of coloured overlays. They are not a magic wand, but may help to stabilise the text reducing the amoun of effort require to read. We used to have pupils assessed through the Irlen Centre, and they would suggest the exact overlay to be used. However, various coloured overlays are available online, and they're not expensive, so it might be worth a punt to buy some and do your own experimenting. Sometimes children were prescribed specs with coloured lenses, and these would sometimes be a different shade or colour to their overlays, this is because the overlays are in direct contact with the text, and obviously specs lenses are a distance from the text. Which is why I think it might be worth just trying them out and finding if any suit your needs. You can also double up, so you may want to combine two colours in order to get a result.
Sorry if this sounds a bit of a faff, but they're quite cheap to buy, and if they help then obviously it's worth the investment.
Good luck, Nic