Thanks all, have reduced spring length by 12mm, top hat from TBT kit has been shortened and is just held captive by closed coil of factory spring, and back to using original HW guide at rear as the coils have closed down a bit too tight for the TBT rear guide

Five shots across my cheap Chinese chrono have it at a smidge over 10ft/lb with H&N FTT, no noticeable twang, but still feels quite sharp (although it was being held at hip, not shouldered)

Wait for daylight for a few aimed shots - see how it 'feels'