I have tried a "without delay" version every 50ms and have managed to get intermittent manchester messages through to my Pro Mini. My atTiny85 is sending on repeat with a 20ms delay between transmissions (nervous that back-to-back tx might get confusing for rx but I am clutching at straws and have no evidence that the delay between tx is useful). The range (with 173mm 1/4 wavelength wire antennae) is about 1m (I need 3m preferably), is hugely dependent on antennae orientation and the reliability is nowhere near good enough but it's progress nonetheless! Thanks Ray.
I will try tx without the 20ms delay next. Does anyone have any thoughts on whether RC_Switch or VirtualWire libraries might be better at this application (i.e. not rely on interrupts or take less time to receive a message)? Can we do anything about the interrupt disabling in Neopixel library? Thanks, Simon