Where and how exactly did you fit it? I can't quite picture how you get it to fit. Do you just push the whole thing into the top hose?
Sorry didnt take any pics, but process simple enough although fiddly.
1. Obtain R5 thermostat (1972-85, its the same for engines 800cc to 1.4, but be sure to obtain the 89oC one).
2. Remove the top hose (from thermostat to top of radiator).
3. Heat the pipe in hot water and smear the inside of one end with washing up liquid.
4. Insert the new stat into the pipe. It's a struggle and you will need some carefuly chosen language, but it will fit. The R5 stat is a strange shape with a brass part that sticks out. Don't worry if it sticks out past the end of the pipe a bit, there is room for it to poke inside the original thermostat housing!
5. Wash the hose carefully to remove the washing up liquid.
6. Refit the hose and refill the coolant (you don't loose very much when removing the hose), then run the engine until hot checking for air.
Is it a permanent fix? You will have your own view. Personally with perhaps a new top hose I don't see why not. A number of cars had the stat fitted in a hose from new so there is no mechanical or design reason why not. The purist will scoff of course, but sometimes just because someone's solution may be different doesn't mean it's wrong.
Given the number of posts on here regards thermostat problems its got to be worth considering.