After the Retroleum test rom diagnosed the newly fitted ic15 as still the fault on the Spectrum practice board, I took it back to the Spectrum4Ever Facebook group and Mark Fixes stuff. Brendan Alford offered up his test rom available at https://github.com/brendanalford/zx-diagnostics to try in the Retroleum which looked like this when testing.

Retroleum test with Brendan Alford test rom:

So IC22 looks like the culprit now. Soldering iron warmed, ic removed, socket fitted and new IC fitted, powered on and got weird patterns, not good. A quick check found some solder splash bridging pins on the board, cleaned off andcthe test rom now did this

It even played a game now 😀