Further research though, and chatting with the staff, shows that the beers are actually being brewed by the members of the Phipps family at Hoggleys Brewery in Litchborough. Apparently there has been a lodging of the company name Phipps Brewery and a plant is ready to start in Overthorpe on the Northants/Oxfordshire border, but they are not yet in a position to brew beers commercially from these premises. The beers at the Northampton Festival are therefore test brews with the festival being the ideal opportunity to ascertain how well they might do in the free market.
I tried both beers on offer: Newbottle Beer at 3.9% and Purston Gold at 4.1%. Both beers suffered slightly from diacetyl but were perfectly drinkable. I preferred the Newbottle which scored 3.4 on the Ratebeer scale whereas the Purston Gold scored 3.0.
Both of them had the diacetyl in the aroma showing signs of being not quite ready, but they were well brewed English ales with a nice balance of hops and malts. They won't set the World alight but they are perfectly drinkable. I was told that the beers had been going down well at the festival, particularly the Newbottle Ale, so here's hoping that the positive feedback from customers at the festival gives them the confidence to start brewing on their own plant, and that Northants has another decent real ale brewery.