I dont know the Mk3. You describe the sealant as Latex. Is it the type of tyre inflation/ sealant aerosol such as 'Holts tyre weld' that has been available for years? Although better than nothing these are limited in their ability to seal punctures.
The Mk 4 comes with a tyre sealant canister that is a much more 'gloopy' and able to temporarily seal some more severe punctures , but still NOT side wall damage that can easily occur if you hit a nasty pothole. The other disadvantage of gloopy tyre sealant is many, if not most, tyre fitters will refuse to to do a puncture repair if its been used. It takes too long to fully remove, and can obscure the site of the puncture.(s) (The last tyre I took in for a puncture repair they found a total of 6 nail penetrations.
) But the simple latex aerosol sealant is much easier to wash off for a puncture repair.
If you have a tyre well, I too would recommend carrying a full sized spare. Even if you have to call out the AA etc to fit it for you.
