A M5 Helicoil might do the trick. You will need to enlarge the hole to 5,2 mm. Go to a mechanic if you're not confident with a repair like this!
Alternative is to glue 15 mm. of threaded rod into the hole with Loctite 638 and use a M5 nut to close the cover.

And do not use silicone, but use something that stays flexible (like Hylomar blue)

Cheers for the rapid reply. I’ll will get onto it. ???? I have some hylomar blue at the ready. 

Managed to find a slightly longer screw as there was enough thread left at the bottom of the hole. Just needed a washer and tightened up nicely. Just need to fill her back up with oil but should be good to go.