I found I had this mini-sheet issued, it seems, to fund the 1990 Stamp World Exhibition in London. I thought the set of PHQ (Postal HeadQuarters) cards in my collection were identical but there are differences even though the subject matter is the same. The mini sheet was issued two weeks later than the individual stamps.
At the time of issue only Ironbridge was a UNESCO World Heritage Site but now all four are listed with the Pontcysyllte Aqueduct (bottom right of the mini sheet) being the most recent.
This is a post for Sunday Stamps, now hosted by Violet Sky at "See it on a Postcard!"