Was getting kinda desperate there for a little while, but was havin fun !
You got it dude. It is the Handley Page HP 115. The HP115 was a slow speed research aircraft designed to investigate the handling characteristics of a very slender delta, flown in the evolution of Concorde !
I was convinced it was from an early car like a Bugatti racer or similar. Couldn't figure out how an 'plane would have such a slab side 'side' to stick some vents in. Then I convinced meself it was an inline like on the DH88...nope... Fairchild Argus....nope...Rapide...nope. Then decided the gills looked too close and quite small for a 'plane so started looking at choppers. First thought, like you , was the Seaking, then having another look, it looked vintage, like when choppers were spindly things.
For a change, instead of a 'close-up'...here's a 'far-up' !
As soon as its recognised...and you shouldn't need Special Branch to look up its family tree (thats a re-leaf !)...I'll show the pic that followed this one.