I can't remember coming across a fancy back teaspoon of this design before, very similar to the 'shell and scroll' patten, but a single flower instead of the scroll.
It's top marked, but only struck with the two marks that you would expect to find on a bottom marked teaspoon such as this. It shows no signs of any additional worn marks.
The Lion Passant appears to me as one of those used at Exeter, as identified by Miles in his article http://www.925-1000.com/a_exeterlions.html
under the 'Anomalies' section, although I am uncertain as to if the base is wavy or there has been a slight distortion over the years.
Presuming that it is indeed Exeter, then the maker would be Thomas Eustace and the dating between 1780-1784, outlining the shift between bottom marking and the addition of the Duty mark.
Other opinions would be welcomed. Also is the fancy back design unusual, or have I been walking around with blinkers on?