There is not going to be a Lantern Festival at Heartwood Forest this year, but there is one at Tring Park instead. This is a completely different site to Heartwood, with expanses of open grassland and well-established woodland around it. It is also just a short walk away from the Natural History Museum at Tring.
http://www.woodlandtrust.org.uk/news/events/tring-lantern-festival/ – Have a look here for more details. Booking is essential for the event.
Tring Park is also a great place to go at this time of year if you want to see some Autumn colour in the trees!