Barnston Riding Centre closed down a couple of weeks ago, unfortunately the voucher is worthless.
Is it a Groupon voucher? If so, I believe you can get a refund if you contact Groupon.
thanks for the update shocked !
It was a great surprise to a lot of people, very sad.
The place is up for sale to anyone who happens to have a spare £1.5million -
http://www.rightmove.co.uk/property-for-sale/property-41525467.html