A guy I used to tile with and myself once drilled in a level (eventually we had a few level lengths done) and put through the holes at either end an adjustable bath foot. So we had a good 6 inches of adjustment and would always know we were tiling level.
We'd use that at floor level when tiling the walls, and if need be we'd prop it up with a stack or two of tiles to give us extra height on the larger wall tiles, and if the tiles started to drop a bit below out datum line we'd use a spanner to adjust it back up to level. Worked perfectly and cost about £3.99 to make.
I guess that has to be something like what you're making, or something that you pin to the wall at any height?
I think you'll find that unless you have a large amount of pre-orders that the dragons will say the market is too small for them to invest. So I'd recommend either getting some info regarding licensing it out to an existing established tiling tool maker, or get a lot of pre-orders that are guaranteed when it's created, if it isn't already.
I love Dragon's Den and watch every one (even the online ones) and time after time they get inventors on that have an idea, but no real return on investment. And they want to make money, not launch a product that they don't know will sell. So make sure you pitch it so it's making them money!
Good luck!
365Drills did a session in the den and you can read about that in the
Tile Drilling forum.
