A found a partial solution, which has the benefit that I can use modified levels!
Unfinished Business plays levels from the HD, so I converted The Caves level from .PHD to .TUB format using the excellent Rview tool. Then I simply renamed it EGYPT.TUB, started a new Unfinished Business game in Glidos, and there was the level in all it's hi-res replacement texture glory! Using UB instead of TR cuts down on one intro movie so it's a bit quicker to load.
Additoinally, I have got hold of a different version of Tomb Raider, and using this method I can test out different texture packs for different versions. But the best thing is that using some custom TR utilities, I can modify the levels to do things I really need -- for example, getting the camera to do a close-up view of Lara. This is essential for getting all the textures to work properly.
Finally, you can see that this opens up the possibility of custom Tomb raider 1 levels having custom high-res graphics.... 😀 😀 😁