Multiconditional numbering of data

G’day excel forum

I’m hoping this is a simple one but it’s really doing my head in at the moment.

I’m trying to find a function to place in cell E7 (see attached spreadsheet) that will incrementally assign a numerical ID to each combination of lower and upper values (Columns C and D) when I drag down (as shown in column H (see attached spreadsheet)).

I'm certain there must be some simple way of doing this but I've been staring at it for hours and at this point my eyes have glazed over and I just can’t seem to see it...

Any help on this one would be greatly appreciated.

Regards
Matt