Yes, the shieldbug Tritomegas sexmaculatus is definitely one to look out for, although I am not sure I agree that Faversham is only a stones thrown from Essex! (pretty much an hour's drive on the M2 from the Dartford Crossing). If it's not in south Essex now, it surely soon will be.
Please note the similarity to the native Tritomegas bicolor which may be found feeding on the aerial parts of white dead-nettle and black horehound, the main hostplants.
The UK Bugs website indicates that Tritomegas sexmaculatus was found during 2011 at two sites in Kent, in both cases feeding on black horehound, the most frequent foodplant on the continent - so the foodplant is not a guide to the species, one needs to look at the more extensively white margins of the pronotum and the much darker (black) wing membrane (http://www.britishbugs.org.uk/heteroptera/Cydnidae/tritomegas_sexmaculatus.html 