Many thanks Paddy - it's great to know what it is !
Sounds like I will need to try to acquire a copy of Wither's list of Evasions along with literature (with decent pictures !) on contemporary counterfeits, as I'm likely to run into other examples (based on my so far 5 and a bit months of collecting). I did wonder if it appeared in "A Journey through the Monkalokian rain forests in search of the Spiney Fubbaduck. (Evasions)" which I believe also covers evasion coins.
Best Regards