Just about to fit a new bottom bracket (slx) and the tool doesn’t fit. I see they have changed this to a smaller diameter. Is there a new specific tool for this or do I need the plastic adaptor (metal tool/ plasic adaptor/ metal bracket seems a really bad idea)?

Yes and yes. 😉

Wasn’t there an adaptor in the box?

as above, adaptor in the bag with mine. plastic tool works great, have used it several times.

Is it the same as new Ultegra, anyone know?

Is it the same as new Ultegra, anyone know?

XTR & Dura Ace use a 39mm OD cup
Ultegra and new XT (MT800) use a 41mm OD cup
Older stuff uses a 44mm OD cup

This.
Park Tool BBT59 16-Notch External Bearing Bottom Bracket Tool 41mm O/Diameter

Also got caught out with this, no adapter with mine either. Improvised with a mile wrench and some rags to stop it scratching Unsuccessfully!). Waiting on CRC to get them back in stock for when it needs to come back off.

that sort of makes sense – my old old HT2 tool was no good for the thin centrelock lockrings you need for thru axles – got a big two ended ring (cackle ye not) spanner and wondered what the other end was for… now I know part of the everything gets tinier and tinier and oh so weannie and sometimes disappears land that is this centuries BB standards

Same happened to me on my new xt. Some seem to come with the wee adaptor ring….mine did not so just got one on eBay for 3 quid. Tl fc25 is what you need if the slx is the same as the xt diameter, this then fits into your old spanner and away you go.