Last of the Series 3s had er... not so good chassis quality/protection so many of them have rotted away. The 2.25 Petrol will usually return between 18 and 22 mpg, but if it's not maintained (i.e. setting timing, points, gaps etc) then the fuel consumption can go through the roof with gutless performance.
The last of the Series 3s as you think do utilise some parts from the very early One Ten production, crossover period. Or rather I should say, the early One Tens used parts from the Series 3 production.