Suitable bolt with level-plug:

Bolt cut and tapered, with spacer and location stub welded on the back of the head:

Showing the effect of the spacer, the adapter is how held at the open end of the socket, allowing the user to press the adapter into the axle plug for maximum effect. Product placement for Halfords unintentional!

Adapter in use. This is on a spare axle in the garage. My drive is on a slope, so I usually back the car onto ramps until it is level, which gives me plenty of room to get underneath to check the level. It also means that the weight of the car is on its springs, so the axle is tucked up in front of the fuel tank, which is what limits the space available for socket drive and attachments. If I jacked the body up at the rear to be level, and let the axle hand on the check-straps, space might not be a problem, but that is a lot more fiddling about than simply driving it onto ramps:

But I still had problems getting enough purchase from the tapered square drive, so decided to modify a plug by welding a 3/4" bolt hex end with a short stub into the square recess:

Comparing modified axle level plug to gearbox level plug - the latter being significantly larger. Measuring the two shows no overlap in the diameter of the tapers i.e. the large end of the smaller plug is still less than the small end if the larger plug ... if you see what I mean:

Modified axle plug with 3/4" hex head, and hopefully no more struggles undoing it. However there is a possible safety implication with the head protruding like this in that in a rear-end shunt the tank could be pushed against the back of the axle, and the hex head puncture it, whereas the original 'flat' plug wouldn't:

A number of suppliers now show this plug with a hex socket for a 3/8" Allen key. However all suppliers quote the original part number - 6K499 but not all show the socket, so they could be the original tapered square socket, you would have to check:

Fitted, as safe as the original plug, but a jolly-sight easier to remove. Using an extension tube (aka box spanner) on the Allen key makes removal even easier, but tighten using just the key on its own to avoid overtightening:

March 2024: Newly-fitted plugs protruding a lot more than mine above. Immediately after this was posted on the MGOC forum I fitted one to a pal's 78 and it was the same. I wouldn't be happy with the drain plug protruding that much given that is is one of the lowest points of the car, my originals are recessed into the casing slightly and barely visible in the above or below: (P Lee)

However Vee's level plug does protrude more: