I want to make a small scaffold about 2.5 metres high with a base about is 2.5 x 1.2 meters- for my self I am about 75kg - Please can you advise What size galvanized tubing do I need ? would 20 x 20 x 1.6mm Galvanised Steel Square Tube Grade 200 be strong enough ?
For our readers on the North American side of the pond, the 20mm x 1.6mm square tube is approximately equivalent to our 3/4 inch 16 gauge square tube. Unfortunately, that tube is much too small for this job. Your life (or somebody's, anyway) is in the balance -- at a standing height of 2.5m, it's not overly dramatic to say that.
For reference, the end frame of the scaffold I'm familiar with stands 2m high and is perhaps 1.5m wide. A second end frame would stand parallel to the first about 2.5m away with two X-shape cross braces fixed between them. That's comparable in size to what you've described. The end frame would be constructed of round tube, perhaps 32mm diameter and 2.0mm wall thickness while the cross braces are round and about 22mm x 1.5mm. (note: sizes estimated from memory; I didn't go out and measure!)
Such a scaffold can readily support two workers, a plank or two of 15-20kg, and some tools or materials as well. It's far stronger than is necessary to support a single 75kg person. But ladders and scaffolds should not be "value engineered" to minimize tubing size -- it's far too easy that down the road its usage will change or grow, it'll be called upon to support more weight than was originally contemplated, and failure could result.