From
The Flight Centre at Halfpenny Green : Private Pilot Training PPL : Commercial Pilot
PRESS RELEASE APRIL 2010 As many of you are already aware Halfpenny Green Flight Centre ceased trading early 2010 and subsequently entered administration. Recently, Jetstream Executive Travel purchased the equipment and property previously owned by Halfpenny Green Flight Centre. To clarify - Jetstream Executive did not purchase Halfpenny Green Flight Centre therefore all debts and liabilities are to be handled by the appointed administrator. The facilities and some of the faces have remained the same which has enabled operations to recently restart. Please contact operations on 01384 221456 for any further questions.
Reading the press release it looks like you voucher will be invalid as Jetstream Executive Travel didn't purchase Halfpenny Green Flight Centre.
I would give them a call and see what they say.