In the loft we have boxes and boxes of old slides. I've seen adverts for these machines that scan them and put them onto either an SD card or straight to your computer. Has anyone got or used one of these and are they any good?

I have an Epson scanner which came with a slide attachment - it's not automatic, you have to load the slides into the holder manually. It holds four slides at a time, which makes doing boxes and boxes tedious, but it works nicely. If you have lots of time available, you might consider similar.

I'm looking at this as my winter project when I can't get out in the garden every day so not too worried about the time it takes.

In that case, and assuming you don't already have a scanner, methinks you could do far worse than find a general purpose scanner that has transparancy abilities.
Once you're done scanning in the slides, you can have a start on all the prints you probably also have

Only a few dozen albums full - nothing major!

Sounds like good advice, thanks