CASTLE OF HORROR, along with its companion TALES OF TERROR (a future post folks), is one of the great oddities of the Starlogged years: regular UK format comic magazines reprinting Marvel material... but not actually published by Marvel UK themselves.
These fall into the gap between the cancellation of the weekly DRACULA LIVES! (the Tomb of Dracula reprints continued in PLANET OF THE APES, THE MIGHTY WORLD OF MARVEL and MARVEL COMIC) and the launch of CHILLER POCKET BOOK which utilised much of the same material from Marvel's US colour comics (Dracula, Man Thing, Werewolf By Night, Ghost Rider and others).
Launched in 1978, these two magazine-format monthlies were published by Croydon-based Portman Distribution and reprinted, under license, material from Marvel's US line of black & white horror magazines (Vampire Tales and Tales of the Zombie). They were initially overseen by Alan McKenzie (latterly of Marvel UK proper).
I stumbled across these titles by accident in the back issue bins and its taken me a while to piece together a full collection and any background information.
I initially assumed that Marvel UK had contracted-out the work because they either felt uncomfortable with the material or they felt it was insufficiently lucrative to bother with (not that it ever stopped the Annex of Ideas reprinting a host of other short-lived duds). Reading McKenzie's website, it seems that the licensing deal was done directly with the States and it took the UK operation a while to spot what was going on. Once they belatedly cottoned on, they complained to head office and the deal was immediately terminated.
To the best of my knowledge, this is the only time throughout the history of Marvel UK (excluding annuals and other books) that Marvel material was licensed-out to another publisher.
That explains why neither ran past five issues although, typically, Marvel UK didn't launch in-house replacements until the short-lived MONSTER MONTHLY, capitalising on the horror movie boom (and replacing Chiller which sunk along with the survivors of the Pocket Books line in 1982), early the next decade.