Another question is, why do I need to TT the supply in the garage? Can't I
just use the house earth?
(My logic is, the earth provided by the electricity company is at least 500m
long (That's where the substation is!)

(Patio is currently being re laid, so if I need to do any rewiring, now is
the time)

Sparks...[/quote]
The simple answer to this question is that unless you are an approved electrical contractor under part "P" you can't do this as sheds and garages are classed as special risk areas. If you wish to do it yourself you must be able to show competence and notify the local building inspectorate before carrying out the work and they will appoint an inspector / tester to verify the work before it is re -energised. This will cost the same as getting an approved electrician to do the job or even more than.