It was taped up in the original styrofoam (I did have the box, but the dampness got to it and that was that). I got the 3 cartridges from eBay I'd ordered in less then a week (very good trader). I let it sit in the upstairs spare room over night (just to make sure it had a chance to "air out"). Plugged in this morning. Fired up - LCD TV tuned in very easily - and cartridges played just fine!
Very happy start to the long weekend, playing a couple of hours of Pacman, Digdug and Defender. I had to repair the Joystick I had found (it had the right leaf switch off-center so wasn't doing anything - manufacturers fault, not damage thankfully) but well, that's one old machine I can tick off as "in working order".
This is the FIRST time I've actually used it since I bought it about 20 years ago - simply because I had no software for it. I'm chuffed...
Now, while the wifes away I'm wondering if I can sneak the Speccy upstairs before she gets home and sees the mess? lol!

Here's a few piccies for anybody interested. Dropbox won't let me directly link to the images to embed them into the post itself, so here's the links as, well, links...
https://www.dropbox.com/s/wcl9dmvclb6lzr2/600XL_InGoodNick.jpg
https://www.dropbox.com/s/do11hfb7jnlo2lf/600XL_Bluescreen_of_life.jpg
https://www.dropbox.com/s/49qi27e4omzz2fs/Atari_Carts_from_Ebay.jpg
https://www.dropbox.com/s/qvooxs5yzpwinsp/600XL_Pacman.jpg