Hi, My car (toyota petrol 1.3) does a smallish mileage in a year say
around 5000 miles, mainly
on fairly short distances around town. what would be the 'time' limit to
reasonably extend oil changes for economy reasons using a semi synthetic oil
?

I suspect a lot of people will immediately say don't economize in this way.
However some forty years ago I had a conversation with a professor of an
engineering college and he said engine oil does not degrade 'time wise' as
fast as most people think ( and importantly as the oil companies would wish
us to
believe). So by how much time could one reasonably delay an oil change with
modern oils?