"No need to buy a plate mic (or more correctly a 'boundary mic') - lying a tieclip mic on its side on a table will do the same job, because that's all a plate mic really is! "

Oh no it isn't!

The capsule is in a little cavity formed between the plate and the flat surface you put it on. Sound enters from all sides and the pressure in the cavity rises and falls accordingly. The mic diaphragm moves as a result.
That's why the thing you call a plate mic is referred to as a PZM as well as a boundary mic. Pressure Zone Mic- from the pressure zone cavity.
Cheers
Dave