Having dug into this a little, the recommended formats for Streetmap are listed here:
https://www.streetmap.co.uk/idlinkto.htm
I've tried out UK Grid format and that works. The "newmap.srf" or "map.srf" format doesn't seem to be listed there, though as gilliwig says, the latter does work.
So my guess is that Streetmap have changed something which has broken the geocaching.com link format. Looking online for random searches, many websites seem to use the "newmap.srf" format, and of course they are all broken.
Interestingly, Streetmap's own "Contact Us" link is also broken. Apart from writing to their Milton Keynes address, I'm stumped for the moment (no, not a clue to one of my hides ). The performance also seems very slow recently, so something must be going on there. Hope it survives, The OS maps provided by the Streetmap link are much better than the Google ones.