In reply to Andy Say:
> Given it is an abbreviation of 'methylated spirits' are you absolutely sure the apostrophe is incorrect?
Yes.
Other than when used for the possessive (eg "Andy's posting") an apostrophe stands in for a missing letter, usually a vowel. For example, in "don't" it replaces the missing/elided "o" of "not". In the much misused "it's" it stands in for the missing "i" of "is" (which is precisely why "it's" as a possessive makes no sense whatsoever).
A single apostrophe is not used to indicate an abbreviation - that's one of the jobs of a full stop (though it's often omitted).
A construction you sometimes come across these days, especially in IT circles, is the <initial letter>-<number of characters in between>-<final letter> form, to indicate a long word. For example, "i18n" as a short form for "internationalisation". Using that method, "methylated spirits" might be expressed as "m16s" - noting that that would include counting the space as a character.
It could also be debated whether "meths" is an abbreviation, a contraction or even a diminutive. Whatever the outcome of that debate, though, "meth's" would never be correct.
> 500 words, please. By Wednesday morning.
Not even 200 words I'm afraid - but then I bet you didn't read it all anyway.