PhiloN, I am interested in your suggestion that this could be an early White Ensign, and presumably the auction house is also satisfied it might date from the right period. I have also seen reference to flags with printed or stencilled designs, although not specifically Royal Navy flags. But if you zoom in on the first image on the auction website using the controls, it looks to me as though the flag is made from one single piece of material - particlularly if you zoom in and drag the image around to follow down the frayed edges on both the right and left, there is no sign of a sewn seam anywhere that I can see. Were flags of this period sometimes made from a single piece of material?
For comparison you can see
a piece of flag believed to be from Nelson's funeral at the NMM here
Boreas, would you like to share the question marks you saw? - Edit: Oops, I see you posted your answer seconds before I posed the question! Thanks.
For another comparison, here is the Minotaur's Union Flag from Trafalgar brought back in 1806 by Master's Mate Stephen Hilton:
It is hard to see in the photo, but the design is sewn from seperate red and white strips of material.
Kester, you will see that the St Patrick's cross is still not positioned perfectly in the Minotaur's flag, although it is very much better than the ensign up for auction.
By the way, the Minotaur's flag, having hung in Selling parish church for many years, is now looking both for a new home and for funding for restoration. If anyone has any suggestions, I would be very pleased to pass them on.