It also strikes me that with a bit of clever rearrangement, you could use a PivotTable to serve up this information. THey would merely have to select the category from a slicer or PivotTable Dropdown, and all the relevant attributes for that category would appear across the top. THen you could still use dropdowns like you have to dynamically work out what is relevent given each attribute header.
By the way, a fully dynamic formula to serve up the validation lists shouldn't present me too much problem, so there will be no need to use that Product Attributes sheet at all.
THat said, it might pay if you could clarify how the Master Data sheet gets populated. Automatically? Or do you (or someone else) have to paste this data in one category at a time from another system?
Jeff thanks for all your comments and interest. As it's your bedtime, I will take a moment to read and consider what you've written, and I'll try to write up something really clarifying.
I really do thank you for your interest and assistance.