Does anyone make accurate reproduction MkV Sten foregrips?

[Vincent]

Would it be acceptable to use the larger capacity Lanchester mag on a Sten which, in theory, should fit or is there some reason why this shouldn't be attempted?

Lanchester magazines do fit in the Sten and Sterling. Of the four I have, only one will take all 50 rounds and function as advertised. The other three are good for about 40 +/- rounds max. Above that number you never know when the floorplate is going to fly off.

The magazines that always work in the Sten are 19 round Indian conversion mags. I tried one right out of the box without cleaning it. It was absolutely filthy. cosmoline went flying and made big mess, but fired all 19.