The Tern which was present at Lisvane Res (19th – 21st at least) and reported as ‘Common’ is in fact a juvenile Arctic Tern [PB, MCP, DRWG]
The pale white primaries & secondaries are consistent with juvenile Arctic, as is the smallish black bill and the faint carpal bar. In juvenile Common, these features are much darker and more pronounced. Also the bill (when seen well) does shows a reddish base, not present on Arctic. [Apologies for poor cropped shot, but it does show the details mentioned.]
Glamorgan Bird Club 
You seem to be cotradicting yourself here by saying the bird shows a reddish base to the bill. Thats why I called it as a common tern as I was pretty certain I saw some red on the bill. All the other evidence certainly points to Arctic so a bit confused.
Alan, Sorry for any confusion: The summary states that in juvenile common tern, the primary & secondary feathering are much darker, ie grey not white, and common has a darker carpal bar. Also as stated ‘if seen well’ shows (on Common Tern) a reddish base – again as stated ‘not present on Arctic’. These terns are notoriously difficult to ID, especially at distance and in immature plumage, hence most are called ‘commic’. I should have made it clearer – No disrespect was ever intended. DRWG.
Alan, fresh juvenile Arctics do have a red base to the bill, which typically quickly turns black by September in the majority. However, the speed of blackening up is variable and some Arctics can still show some red at the base at this time of year. In contrast, all juvenile Commons have a much more obvious red base at this time of year. Hope this helps, Barry
Thanks for your help guys. A quick visit this lunch time produced no terns thank goodness!!!