The reduction in open days is due to building works nearby, and works inside the building itself.
For this is a 600 year old barn, but a huge impressive barn, and described as the “Timber Cathedral of Middlesex” by John Betjeman.
As there are few open days this year, it is worth putting a suitable note in your diary to make a trip in advance.
In addition, should you be so inclined, there is a nearby model steam railway open on several dates that coincide with the Barn, and it is a modestly pleasant walk along rural roads just to the north of Heathrow airport via the village of Sipson to get between the two venues. Or there are buses, although it will involve a couple of changes.
The barn is open 10am to 5pm from May to Sept (11am-4pm in April and Oct), while the railway is open 2pm to 5pm (except 14th Sept, when it’s noon to 5pm).
Oh, and if you do arrive by tube at Heathrow, why not take a detour to use the Heathrow pods?