On Nick's blog, someone using the pseudonym IJ commented that this question was disputed and led to a supplementary being used. Dave P added that "[during] a city break in Portsmouth earlier this year, I failed to notice that the city was on an island – although Wikipedia says it is".
I can further add that during our game, this assertion was questioned but we took the setter's word for it.
The island in question is Portsea Island; Wikipedia's exact words are "Portsmouth is situated primarily on Portsea Island and is the United Kingdom's only island city, although parts of it have expanded onto the mainland in recent years ... The island is separated from the mainland by Portsbridge Creek which is crossed by three road bridges ([including] the M275 motorway ... ), a railway bridge, and two footbridges".
Portsbridge Creek is less than 100 yards wide, and significantly narrower in some places (e.g. where Portsbridge Roundabout connects the A3, A27 and A397 roads). Portsea Island separates Portsmouth Harbour (to the west) and Langstone Harbour (to the east). Beyond Langstone Harbour is Hayling Island, with Chichester Harbour east of that.
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