I can't see the earth lead getting contaminated with oil being the problem. An easy answer is to get some metal brading, and connect it between a good metal part of the engine, under a nut and washer, and a similar part of the body shell. This will act in the same way. If you've a set of jump leads, use one of them for a quick test, one end clip on the starter motor body, the other on a bare metal bodyshell area, or the negative terminal of the battery. (make sure its the negative, or you might get a bit of a hair raising experience.
It's more likely that the internal copper contacts have worn in the starter motor. Should be less than a tenner to replace them, usually an ebay search will work for you to buy. Here's the link for an estima starter motor job, this should be very similar.
https://estima-motoring.co.uk/Forum/index.php?topic=151.0