Tried the link again, then www.marksand spencer.com then www.marksandspencer.co.uk, all worked although the posted link is the only address that takes you straight to the train set.
Just had another go at this and may have gone some way to explaining what's happening. If using the M&S .com or co.uk I went to their web pages OK but couldn't find the train set anywhere so I clicked on the link again and sure enough straight to the relevant page but if you copy and paste the product code into their search space then you get the 'sorry, don't have it' message. This applies no matter how you arrive at the site, even on the page that actually shows you the model. Strange.