Toshirô Mifune did not appear in Those Magnificent Men in Their Flying Machines or How I Flew from London to Paris in 25 Hours 11 Minutes (1965). Harold is evidently thinking of Yûjirô Ishihara.
Some of Steptoe's Seven are clearly much younger stuntmen in aged make-up and costumes.
(25:30) Frankie's grazed left eye appears to appear out of thin air between shots.
(26:08) The flying wires used to propel one of Frankie's goons around in the air is very visible; this is also true of the one attacked by Delilah.
At around 2:17 in a scene in the yard, where Harold is making fun of Albert for being lazy, you can see a wall, painted on to cloth, flapping like a flag.