@retrolover, please check this video out of another game I made - it shows you how to do this and what tool to use. It was another Spectrum game, but you needed to mod it.
Please check the chapters and you want the modding one on the video.
But we must talk about this game...
@marco-leal-zx, this is awesome what you have done it really is. There will be a video on my channel for this on 3rd January as there is already a couple in the pipeline but thank you for this - this is a cracking recreation - you have some awesome skills here.