I had two similar salter scales (model 1051) which were very difficult to turn on/off after some use. Changing batteries and licking fingers didn't work. The fix is easy, just clean the switch(es) with a solvent to remove the almost invisible coating (patina) that builds up from the grease on your fingers. Both my scales are now working like new. I used a tiny bit of 100% alcohol on a q-tip (cotton bud) and rubbed the touch-switches for less than a minute. I'm sure lots of other solvents will also work.
I discovered this when taking the scales apart to fix ithem and noticed that touching the inside surfaces of the touch-switch (where there was no grease layer) successfully turned the scales on.