Wolfie,
The 1:72 scale came about by the use of the ratio 1" = 6 feet, or in other words, 1" = 72".
Considering that 1" is approximately the same as 25mm (actually 25.4mm), then stikpusher is correct in stating that 25mm scale is the best alternative for you.
By the same rationale/calculation, 20mm is equivalent to 0.79". If 0.70" = 72", then the scale is actually about 1:90, or a touch smaller than HO (1:87 scale).
Incidentally, you can also happily use OO scale railway buildings, vehilces, animals & accessories with your 1:72 scale dioramas. The OO scale is 1:76, so the differences are barely noticeable.