Hi can anyone tell me what would be classed as normal oil consumption

When it's not excessive

When youre topping up rather than "Using" oil. It really depends on how you ride, the amount of miles you're doing etc. Obviously check for oil leaks - the cam cover gasket sometimes weeps. After that check the plugs, see if they're oily that could be rings / bore wear. Look for a puff of smoke on start up suspect the valve oil seals. Check the crank breather pipe isn't pinched or kinked.
The only other thing would be to say these bikes are 25 years old, they weren't built to the tight tollerances of today's engines and so they use oil to fill those tollerances.