I'm not a programmer. The Next documentation states that, by default, it can handle a tileset of up to 512 tiles if the ULA space is utilized. And Mike Dailly just wrote a blog post about ULA, TILEMAP, and sprite rendering priority using BLEND modes.
Viewing post in Astro Warrior port for Spectrum Next comments
I don't have the docs to hand but the game uses a word sized tilemap which reduces the space for tiles to (I think) 432 vs the SMS 448. Not a huge difference but just needs working around. I did briefly look at Stencil/Blend modes but did quite understand them, will have a look at Mike's blog to see if it helps.