So, it turned out to be Uronema

Overnight the wound went from what looked like it was healing and definitely less red to what you can see in the photo. We took her out immediately and put her in the QT and did what we could to treat, including FW dip. Sadly she died the following night.
The other 3 fish and all inverts are looking great, no signs of anything going on both physically and behaviourally. I know this tank is now effectively toast and we will need to restart. We plan to do a bit of a shuffle, move them into another tank and won’t ever introduce anything else (probably our QT and buy a new QT as it is probably infected now too). As for the DT, we plan to bleach and fully break it down and start again.
Question is the LR and inverts… I’ve read that they can’t carry Uronema and that a good rinse off should get rid of anything that may be ‘attached’ and to possible bleach the LR??!! I’ve also read completely contradictory info too I’m a little lost here!
Any advice would be so appreciated!
Side note - J&L took full responsibility, admitting that the Uronema came from them and have likely lost a fair few fish now too (I’m not sure if they filter multiple tanks on the same system)? Either way - definitely avoiding them now so if you have any recommendations on suppliers we would love to know!