I'm presuming you've got a moss problem?

Ya - lots of it


My questions a
1. How often can you do the whole, fork the lawn, brush in sand
process? I suspect I could do it more than once a year but I also
suspect that at some point it will start to do more harm than good.

Unlikely to do much harm if you do it 4 times a year. Football pitches
are treated regularly.

I suspected as much

4. Are the shoes you can buy with the spikes on them which are meant
to aerate your lawn any good or just a gimick?

If you are a clown, then they may be good.

Hehe

A good lawn takes constant work to get right. What sand are you using -
builders sharp sand? or the fine silver sand?

Sharp sand - how much should I spread at a time i.e. how deep should
it sit on the lawn?

and thanks for your time