Thanks - I had a feeling that that was the answer. Essentially, electrically, they are "just a relay" but they are manufactured to high standards, have some special features to "guarantee" reliability, and are certified as such. My impression is that some people fit them, probably a majority do not, but you pays your money and you takes your chance - in a home workshop environment, anyway. I was a bit put off when a quick google search took me to the Farnell site and saw prices from £100+VAT and up. Then I took a look at eBay, and that makes it look a bit more sensible! In the overall scheme of things, especially if I go down the external motion control board route, it's not going to add an enormous amount to the overall cost.