It was only edged from the top spot by Church Fenton in North Yorkshire, which registered a 28.6C temperature.
Despite the soaring heat, Thomastown didn't reach Castlederg's Thursday temperature of 28.8C – the hottest in the UK.
The average temperature in Belfast yesterday was 24C. Coastal areas, such those as in and around the Giant's Causeway, were cooler, with the average temperature here recorded as 18C.
While cooler air is moving over Northern Ireland today, the message is don't put away the barbecue just yet.
Belfast is expected to reach 19C while the western part of the province will drop to 21C and, according to the forecasters at Meteo Group, this will last right through to the middle of next week.
The long-running heatwave brought welcome sunshine for thousands of parading Orangemen and supporters yesterday.
And for those enjoying a day off, the beaches were packed yet again.
One group of businesses delighting in the great weather are ice-cream parlours.
Gary Bartlett who runs Mauds ice-cream shop in Bangor said he was run off his feet before, during and after the Twelfth parade.
"It was definitely one of the best business days of the year. It has been great since the weather improved. We have had a lot of families in getting cones and tubs to eat later at their barbecues," he said.
Sloan's Gas Energy Centre in south Belfast also reported a notable increase in barbecue sales over the past week.
However, a warning was issued yesterday about the dangers hot weather can pose for elderly people. North Antrim Ulster Unionist Robin Swann MLA said the whole community needed to help older people combat this threat.
"Just as many elderly people had to overcome the atrocious winter conditions, when they were forced between eating and remaining warm, so, too, in the current heat wave, the elderly are also at risk from heat stroke and dehydration. We, as responsible citizens, have a role to play. I appeal to the community to become the eyes and ears of the elderly," he said.
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