Hi all I've got a 1.3 spi and seem rubbish on mpg. it's had full service temp does get to 90 in car seems to take a while tho. so replaced temp sensor removed stat and it opened and closed as it should in boiling water. Temp also goes down from 90 if heaters go on when shouldn't it stay at 90 whatever. So anyone got any idea's cheers in advance

Saying you get 100 miles to £20 of fuel isn't the most accurate way to calculating the MPG.

You really need to brim the tank, do 100 miles or whatever and brim it again at the same pump if you can and take a note of the mileage done and litres put in. Also take into account the cold weather does effect MPG just like your driving style and type of driving.