As you'll see below, Tomy paid M-UK £400 to run the insert in 40,000 copies of Indy.
The invoice was a two-sheet affair so I've scanned both. The first scan includes the lower-half of the second page.
The second scan is the second page.
The ZOIDS insert also appeared in RETURN OF THE JEDI (84) and SPIDER-MAN (620) weeklies dated 26 January 1985.
A second Zoids insert ran in September 1985, by which time Indy had been cancelled.
How did I get these? I stumbled across some back issues of INDIANA JONES MONTHLY in a (now defunct) London comic book store a few years ago. They weren't in terribly good condition (battered and hole-punched) but I needed them to fill some gaps. And they were cheap.
I noticed these invoices paper-clipped to the interior of this issue, as well as a freelancer payment for a text feature attached to another issue, which seemed pretty unusual. Another reason to plonk down the required 50p an issue.
I can only assume that these were M-UK file copies, probably held by the advertising sales department, that somehow escaped the business. Possibly during an office spring clean or - maybe - when Marvel moved from Redan Place to Arundel House or (more likely) when they decamped from London to Kent.