I don’t know who the artist is on the cover of Gigantic Annual #5 (1964), but I reckon he does a great job of mimicking John Sikela's style.
It’s clearly not Hart Amos. I posed the question some time ago to Kevin Patrick, and he advised it’s not Peter Chapman, who did some K.G. Murray covers for DC material such as on the Color Giant #2. Nor is it John Dixon or Yaroslav Horak.
Anyone care to vote for Peter James or Kevin Hardacre - a couple of names Kevin threw into the mix - or anyone else as likely candidates?
Regardless of the identity of the artist, the style is certainly consistent with the Amos template set up for Mammoth Annual, Gigantic Annual and Colossal Comic.
For the record, my copy contains the following rebound issues:
Century Comic #86, July 1963
The Hundred Comic #82, September 1963
All Favourites #37, June 1963