The engine tail pipes are too thin for even a Mk 4. Is that the result of a casting problem, or was the original an even older Mark, with the narrower jet pipes?
Likely that it's either an F.1, as suggested in the previous post, or one of the early F.3 aircraft. Both were powered by the Welland, which had a considerably narrower jetpipe than the Derwent that powered subsequent aircraft:
